JP2003060292A - Wavelength stabilized light source, wavelength control device and wavelength measurement device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To realize highly accurate wavelength stabilization with an inexpensive configuration. SOLUTION: The collimator lens 12 is a DBR-LD.
The rear output light of 10 enters the polarizing prism 20 via the collimator lens 12, the plane beam splitter 14, and the birefringent plate 18. The polarizing prism 20 separates the output light from the birefringent plate 18 into two polarized light components, one polarized light component is supplied to a photodiode (PD) 22, and the other polarized light component is supplied to a PD.
24. The birefringent plate 18 is arranged so that the slow axis and the fast axis are inclined by 45 ° with respect to the polarization direction of the laser beam from the plane beam splitter 14. Matches the polarization direction of the beam. The output ratio calculation circuit 26
The output ratio of the PDs 22 and 24 is calculated, and the calculation result is applied to the control circuit 28. The control circuit 28 includes an output ratio calculation circuit 26
, The current injected into the DBR of the DBR-LD 10, that is, the reflection wavelength of the DBR is controlled so that the output of the output ratio calculation circuit 26 becomes equal to a predetermined value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength stabilizing light source that stabilizes an oscillation wavelength, a wavelength control device that controls the oscillation wavelength of a variable wavelength light source, and a wavelength measuring device that measures the wavelength of the light source.

[0002]

2. Description of the Related Art In the field of optical communication in recent years, a wavelength multiplexing technique for simultaneously transmitting a large number of signals having different wavelengths in one fiber has made a great contribution to the expansion of communication capacity. In general, the narrower the signal wavelength interval, the higher the spectrum utilization efficiency and the communication capacity.

In order to narrow the signal wavelength interval, it is necessary to advance the wavelength multiplexing / demultiplexing technology and stabilize the laser oscillation wavelength of the coherent light source. The signal light source is a light source (backup light source) that outputs laser light of the same wavelength in case of failure.
Need to be prepared. Since it is inefficient to prepare a backup light source for each signal wavelength, if a wavelength tunable light source is used as the backup light source, a smaller number of backup light sources than the number of signal wavelengths will suffice. The wavelength stabilization technique for controlling to a desired value becomes extremely important.

Of course, the wavelength tunable light source and the technology for stabilizing the wavelength are extremely important not only for optical communication applications but also for optical information processing and optical measurement.

As a light source for ordinary optical communication, optical information processing and optical measurement, a coherent light source that oscillates a laser with a single wavelength and can change the oscillation wavelength is used. Structurally,
There are an external resonator type LD in which a laser diode is arranged in a Fabry-Perot resonator, a distributed feedback laser (DFB-LD) and a distributed Bragg reflector type laser (DBR-LD) in which a diffraction grating is embedded in the laser diode. In order to change the oscillation wavelength, the reflection wavelength of the reflector may be changed. Specifically, in the external resonator type LD, one of the external reflectors is used as a diffraction grating and its angle is changed to change the reflection wavelength. DFB-LD and DBR-
In the LD, the reflection wavelength of the distributed diffraction grating is adjusted by changing the internal refractive index according to the injection current or the temperature.

Since the external resonator type LD has a wide selection wavelength range of the diffraction grating, it has an advantage that the oscillation wavelength can be continuously changed over a relatively wide wavelength band in which the LD has a sufficient gain. There is. On the other hand, since a diffraction grating having a large size is required, there are various drawbacks. That is,
Since the resonator length itself is long, the size becomes large. An optical coupling system such as a lens is required for optical coupling between the diffraction grating and the LD, which complicates the structure. Since the resonator length is long, it is easily affected by external vibration and environmental changes such as temperature changes.

In the DFB-LD, two distributed diffraction gratings are used.
It is divided, and an electric current is separately passed to each of them, and the injection current value is adjusted to change the internal refractive index, thereby changing the reflection wavelength of the diffraction grating. It is also possible to change the internal refractive index by changing the temperature of the LD itself and change the reflection wavelength of the diffraction grating. Since the DFB-LD has substantially the same size as the LD, it has advantages that it is small and the oscillation wavelength is stable. On the other hand, in the DFB-LD, it is difficult to independently control the injection current required to obtain a sufficient laser output and the injection current for changing the wavelength. Further, when the wavelength is controlled by temperature, the change in the refractive index with respect to temperature is not so large that the wavelength tunable band is narrow.

In the case of the DBR-LD, since the reflector is arranged in the passive waveguide region, the injection current for adjusting the output (gain) and the injection current for changing the reflection wavelength can be controlled independently. Yes, the variable range of the injection current for changing the reflection wavelength can be made large, and the DFB-L
The wavelength variable band can be expanded as compared with D. DBR-LD
In the case of, in order to expand the wavelength variable band and improve controllability,
A structure using a superstructure distributed reflector, a sampled diffraction grating reflector, or the like has also been proposed.

The conventional variable wavelength L including the above-mentioned conventional example
In order to stabilize the oscillation wavelength, each D takes out a part of the output light, measures the oscillation wavelength, and controls the oscillation wavelength to a desired value. For such wavelength control, it is necessary to detect the oscillation wavelength by using an optical element having some wavelength sensitivity.

For example, a desired wavelength interval (for example, about 2n)
m) via a Fabry-Perot etalon (FP etalon) with a free spectral width (FSR) of DBR-L
There is a configuration in which the laser output light of D is input to the light receiving element and the injection current of the DBR of the DBR-LD is controlled so that the output current of the light receiving element becomes maximum. By controlling the injection current as described above, the oscillation wavelength of the DBR-LD matches any wavelength grid of the FP etalon. The absolute oscillation wavelength can be accurately estimated by checking the correspondence between the injection current and the oscillation wavelength separately and applying the current injection current to the correspondence table.

A configuration has also been proposed in which the output light of the wavelength tunable laser is made incident on the interference light filter and the oscillation wavelength of the wavelength tunable laser is controlled by the output power ratio of the transmitted light and the reflected light thereof (for example, Japanese Patent Laid-Open No. 2000-242242). 2000-12968 and JP-A-2000-22259).

[0012]

In the conventional example using the FP etalon, the oscillation wavelength of the DBR-LD can be stabilized at a discrete wavelength at which the transmittance of the FP etalon is maximized. In order to detect that the amount of transmitted light is maximum, it is necessary to apply dither to change the oscillation wavelength, and the stability of the wavelength is determined by the dither range. Furthermore, the correlation between the injected current amount and the oscillation wavelength may change sequentially due to changes in the LD over time and environmental changes, so in the unlikely event that noise such as electrical surge causes the oscillation wavelength to hop to another peak. When it does, it becomes impossible to determine the absolute wavelength.

Also in the conventional example using the output ratio of the transmitted light and the reflected light of the interference light filter, the absolute wavelength can be estimated by examining the correlation between the injection current and the oscillation wavelength in advance. Since the wavelength can be continuously stabilized by the output ratio of both the transmitted light and the reflected light, there is no problem even if the transmission peak of the optical filter deviates from the desired wavelength grid. When the transmission characteristics of the optical filter have no peak, the output ratio of the transmitted light and the reflected light has a one-to-one correspondence with the oscillation wavelength, so that the absolute wavelength itself can be detected. Since the interference light filter is a resonator, its transmission / reflection characteristics change sharply near the transmission peak and change gently near the valley.

Incident light power and transmission of a general FP etalon
Overlight power and reflected light power P₀, P_tAnd P
_rThen, P_t= P₀/ (1 + Ksin^Twox) P_r= P₀Ksin^Twox / (1 + Ksin^Twox) However, K = 4R / (1-R)^Two x = (2πnd / λ) cos θ Where R is the reflectance of the mirrors on both ends and n is the refraction of the etalon.
Where d is the Fabry-Perot cavity length (ie the thickness of the etalon)
And θ is the angle of incidence.

[0015] Therefore, the output ratio obtained by dividing the difference between the two in the sum, and _{(P t -P r) / (} P t + P r) = (1-Ksin 2 x) / (1 + Ksin 2 x) (1) Become.

As an example, the center wavelength is 1.55 μm, and the transmission
Overwavelength spacing is 20 nm, n is SiO _TwoWith the refractive index of glass
1.45, R 90%, incident angle 0 ° (normal incidence)
FIG. 9 shows the transmission / reflection characteristics and the output ratio in such a case. Vertical
The axis shows the output ratio shown in equation (1), and the horizontal axis shows the wavelength.
You Thus, especially when using FP etalon
The wavelength detection sensitivity deteriorates near the light valley. Yo
If you try to obtain a response that is closer to linear, the contrast ratio
As a result, the wavelength detection sensitivity deteriorates overall, while
To maintain a large trust ratio,
The detection sensitivity will change significantly. To improve this
May combine a plurality of FP etalon filters
However, this complicates the structure and increases the loss.
It

In addition, the FP etalon filter usually uses a dielectric multilayer mirror to reduce the loss of the transmission peak, but it is necessary to control the thickness of each multilayer very precisely.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and is a wavelength-stabilized coherent with a simple structure, low loss, high contrast ratio and nearly linear response. It is intended to provide a light source, a wavelength stabilizing device, and a wavelength measuring device.

Another object of the present invention is to provide a wavelength-stabilized coherent light source, a wavelength stabilizer and a wavelength measuring device which can be manufactured at low cost without the need for a precise thin film forming process such as a dielectric multilayer mirror. And

[0020]

A wavelength-stabilized light source according to the present invention comprises a resonator having a reflector whose reflection wavelength can be changed, and a tunable light source composed of a laser medium arranged in the resonator. The output light of the variable wavelength light source is input, and the birefringent plate having an optical axis inclined by a predetermined angle with respect to the polarization direction of the output light and the output light of the birefringence plate are separated into two polarization directions orthogonal to each other. A polarization separator, a first light receiving element that receives one polarization component separated by the polarization separator, a second light receiving element that receives the other polarization component separated by the polarization separator, An output ratio calculation circuit that calculates a predetermined output ratio from the outputs of the first and second light receiving elements, and a control circuit that controls the reflection wavelength of the reflector to a predetermined value according to the calculation result of the output ratio calculation circuit. It is characterized by doing.

The wavelength control device according to the present invention is a wavelength control device for controlling the output wavelength of a wavelength tunable light source, wherein the output light of the wavelength tunable light source is input and a predetermined angle is formed with respect to the polarization direction of the output light. A birefringent plate having an inclined optical axis, a polarization separator that separates the output light of the birefringent plate into two polarization directions orthogonal to each other, and one polarization component that is separated by the polarization separator One light receiving element, a second light receiving element that receives the other polarization component separated by the polarization separator, and an output ratio calculation that calculates a predetermined output ratio from the outputs of the first and second light receiving elements. It is characterized by comprising a circuit and a control circuit for controlling the reflection wavelength of the reflector to a predetermined value according to the calculation result of the output ratio calculation circuit.

A wavelength measuring device according to the present invention is a wavelength measuring device for measuring a wavelength of a light source, wherein an output light of the light source is inputted, and an optical axis tilted by a predetermined angle with respect to a polarization direction of the output light is set. A birefringent plate having, a polarization separator for separating output light of the birefringent plate into two polarization directions orthogonal to each other, and a first light receiving element for receiving one polarization component separated by the polarization separator. A second light receiving element for receiving the other polarization component separated by the polarization separator, and the first light receiving element
And an output ratio calculation circuit that calculates a predetermined output ratio from the output of the second light receiving element, and a calculation result of the output ratio calculation circuit.
It is characterized by comprising a collation circuit that collates with the relationship between the output ratio and the wavelength measured in advance.

With such a structure, the wavelength can be measured, controlled, and stabilized with high accuracy and with a low cost structure.

Further, a temperature sensor is provided, and the birefringent plate is
By comprising the first birefringent material and the second birefringent material whose birefringence amount is adjusted according to the temperature detected by the temperature sensor,
Be stable with temperature.

Preferably, the birefringent plate comprises a first birefringent material and a second birefringent material having a temperature change that cancels a temperature change of the birefringent quantity of the first birefringent material. It consists of a refraction material. Alternatively, the birefringent plate is composed of a first birefringent material and a second birefringent material showing a change in birefringence amount having the same sign with respect to a temperature change, and an ordinary ray axis of the first birefringent material. And the first and second birefringent materials are arranged so that the extraordinary ray axis coincides with the extraordinary ray axis and the ordinary ray axis of the second birefringent material. With such a configuration, the amount of temperature change can be reduced, and the accuracy of wavelength measurement, wavelength control, and wavelength stabilization is improved.

[0026]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows a schematic block diagram of an embodiment of the present invention. 10 is an LD whose wavelength can be changed, for example D
BR-LD, which outputs laser light to the front and the rear.
The laser light output to the front is used for main purposes. The collimator lens 12 collimates the rear output light of the DBR-LD 10 into a parallel beam, and the plane beam splitter 14
Incident on. The plane beam splitter 14 splits the laser beam from the collimator lens 12 into 1: 9, and the former is a photodiode (PD) 16 and the latter is a birefringent plate 1.
It is incident on 8. The photodiode 16 is used to monitor the output power of the DBR-LD 10, as described later.

The output light from the birefringent plate 18 is used as the polarization prism 2
It is incident on 0. The birefringent plate 18 is made of, for example, yttrium vanadate (YVO ₄ ), and is polished to have a predetermined thickness (for example, 556.9 μm). The polarization prism 20 is, for example, a Wollaston polarization prism.
The polarization prism 20 separates the output light of the birefringent plate 18 into two polarization components, and one polarization component is input to the photodiode 2
2, the other polarization component is incident on the photodiode 24.

FIG. 2 shows the relationship between the polarization direction of the laser beam incident on the birefringent plate 18, the slow axis and the fast axis of the birefringent plate 18, and the optical axis of the polarization prism 20.
2A shows the polarization direction of the laser beam incident on the birefringent plate 18, FIG. 2B shows the directions of the ordinary and extraordinary ray axes of the birefringent plate 18, and FIG. The optical axis of the polarizing prism is shown. In the case of YVO ₄ , the ordinary optical axis is the slow axis and the extraordinary ray axis is the fast axis. As can be seen from FIG. 2, the birefringent plate 18 is arranged such that the slow axis and the fast axis are inclined by 45 ° with respect to the polarization direction of the laser beam from the plane beam splitter 14, and
The optical axis of 0 coincides with the polarization direction of the laser beam incident on the birefringent plate 18. That is, in this embodiment, DBR-L
The monitor light of D is equally divided into two orthogonal polarization components, and each polarization component is given a phase difference according to the wavelength, and is then combined again.

The output ratio calculation circuit 26 calculates the output ratio of the photodiodes 22 and 24, as described later, and applies the calculation result to the control circuit 28. The output of the photodiode 16 is also applied to the control circuit 28. The control circuit 28 controls the output power of the DBR-LD 10 to a predetermined value according to the output of the photodiode 16, and the DBR-LD 10 according to the output of the output ratio calculation circuit 26 so that the output of the output ratio calculation circuit 26 becomes equal to the predetermined value. Control the injection current into the DBR, that is, the reflection wavelength of the DBR.

Calculation result of the output light calculation circuit 26 (output ratio)
In view of the fact that the relationship between the wavelength and the wavelength of the DBR-LD 10 varies depending on various factors such as elapsed time and ambient temperature, the following is preferable. That is, the calculation result (output ratio) of the output light calculation circuit 26 and the DBR-LD1
The relationship with the wavelength of 0 is measured in advance, and the measurement result is stored in the lookup table 30 as a correspondence table. Then, the control circuit 28 applies the output of the output ratio calculation circuit 26 to the look-up table 30 to confirm the current wavelength, and controls the injection current into the DBR of the DBR-LD 10, that is, the reflection wavelength of the DBR according to the result. To do.

The wavelength measuring principle of this embodiment will be described. Figure 2
As shown in, the birefringent plate 18 is installed such that the optical axes (slow axis and fast axis) are inclined by 45 ° with respect to the polarization direction of the backward output light of the DBR-LD 10, and the polarization axis of the polarization prism 20 is , 45 with respect to the optical axis of the birefringent plate 18.
It is installed at an angle.

From the plane beam splitter 14 to the birefringent plate 1
The laser light entering 8 is separated into a polarized component along the slow axis (slow component) and a polarized component along the fast axis (fast component) in the birefringent plate 18, and propagates at different speeds. A phase difference corresponding to the speed difference occurs between the slow component and the fast component. The polarization axis of the polarizing prism 20 is 45 with respect to the optical axis of the birefringent plate 18.
Because of the inclination, the polarizing prism 20 has a birefringent plate 18
The slow component and the fast component of are combined in two orthogonal axis directions inclined at 45 °, and one directional component is supplied to the photodiode 22 and the other directional component is supplied to the photodiode 24.

In general, the birefringent plate has wavelength dispersion of the refractive index determined by its material, and the refractive index wavelength dispersion of the slow axis and the refractive index wavelength dispersion of the fast axis are different. However, by appropriately selecting the material, it is possible to obtain the birefringent plate 18 whose wavelength dispersion of birefringence is continuous and monotonic with respect to the wavelength within a desired wavelength range. In that case, the phase difference generated between the slow component and the fast component also changes continuously and monotonically with respect to the wavelength. When such a birefringent plate 18 is adopted, the power ratio of two different polarization components of the polarization prism 20 changes monotonously with wavelength. Photodiodes 22 and 24 and output ratio calculation circuit 2
6, the power ratio can be calculated, and the control circuit 28
According to the calculation result of the output ratio calculation circuit 26, DBR-LD1
The reflection wavelength of the DBR of 0 is controlled to a desired value.

The birefringent plate 18 is YVO_FourAlways consists of
Refractive index n for rays and extraordinary rays _oAnd n_eIs a cell
According to Meyer's empirical formula,   n_o(λ) = 3.77834 + 0.069736 / (λ^Two−0.04724) −0.0108133λ^Two   n_e(λ) = 4.59905 + 0.110534 / (λ^Two-0.04813) -0.01226762λ^Two Given in. λ is a wavelength in μm. Up
From the formula, birefringence Δn = n _e-N_oCan ask
It

The intensity of light incident on the birefringent plate 18 is I, the polarization direction is the y direction, and the propagation direction is the z direction. If the term representing the propagation in the z direction is omitted, the x component of this incident light intensity
The I _x and y direction component I _y is expressed as I _x = 0 I _y = I exp (jωt). However, ω is the angular frequency of the incident light.

If the fast axis of the birefringent plate 18 is inclined by θ from the y direction to the x direction, the fast component I _fast and the slow component I of the incident light on the birefringent plate 18 will be described.
Each of _slow is I _fast = I cos θ · exp (jωt) I _slow = I sin θ · exp (jωt).

Assuming that the thickness of the birefringent plate 18 in μm is t, the phase difference Δφ between the fast component and the slow component when exiting the birefringent plate 18 is Δφ = 2π · Δn · t / λ = 2π {n _o (λ) -n _e (λ)} t / λ (2). Since the phase of the slow component of the outgoing light of the birefringent plate 18 is delayed by Δφ with respect to the phase of the fast component, when the fast component is taken as a reference, I _fast = I cos θ · exp (jωt) (3) I _slow = Isin θ · exp {j (ωt + Δφ)} (4)

The optical axis of the polarizing prism 20 is the birefringent plate 18
Since it is inclined by θ with respect to the optical axis of
Output light I _'x and the output light I in the y-direction' _y in the x direction component of _{0, I 'x = Icosθ · sinθ} · exp (jωt) -Icosθ · sinθ · exp {j (ωt + △ φ)} (5 ^{_{) I 'y = Icos 2 θ}} · exp (jωt) + Isin 2 θ · exp {j (ωt + △ φ)} and comprising (6). Here, when θ is 45 ° (= π / 4), I ′ _x = Iexp (jωt) {1-exp (jΔφ)} / 2 = Iexp (jωt) {1-cos (Δφ) − jsin (Δφ)} / 2 (7) I ′ _y = Iexp (jωt) {1 + exp (jΔφ)} / 2 = Iexp (jωt) {1 + cos (Δφ) + jsin (Δφ)} / 2 (8 ).

The light I'of the x-direction component is input to the photodiode 22.
_{When x} is incident, the photodiode 24 receives light of y-direction component.
Suppose _I'y is incident. Photodiodes 22, 24
Since the power of the incident light is detected, when the outputs of the photodiodes 22 and 24 are P _x and P _y , P _x = P ₀ {(1-cos (Δφ)) ² + sin ² (Δφ)} / 4 = P ₀ (1-cos (△ φ) / 2 (9) P _y = P ₀ {(1 + cos (△ φ)) ² + sin ² (△ φ)} / 4 = P ₀ (1 + cos (△ φ) / 2 (10) P ₀ = I ² .

[0041] Therefore, the output ratio obtained by dividing the difference between the output of the photodiode 22, 24 in _{sum, (P x -P y) /} (P x + P y) = - a cos (△ φ) (11) . The output ratio calculation circuit 26 uses the photodiode 2
From the outputs of 2 and 24, (P _x −P _y ) /
Calculate (P _x + P _y ).

When the thickness t of the birefringent plate 18 is appropriately selected, it is possible to detect a wavelength of high contrast within a desired wavelength range. For example, considering the case of stabilizing the oscillation wavelength of the DBR-LD whose wavelength is variable between 1.545 μm and 1.555 μm, t = 556.9 μm may be set. Figure 3
The relationship between the wavelength and the phase difference between polarizations Δφ at t = 556.9 μm is shown. The horizontal axis represents wavelength and the vertical axis represents the phase difference Δφ between polarized lights. As can be seen from FIG. 3, the phase difference Δφ between polarized lights changes almost linearly with wavelength. Therefore, the output ratio shown in Expression (11) changes substantially according to the trigonometric function with respect to the wavelength. FIG. 4 shows an example of changes in the output ratio shown in Expression (9) according to the present embodiment and the output ratio in the conventional example shown in Expression (1) with respect to wavelength.

As can be seen from FIG. 4, the output ratio of this embodiment is closer to linear in the same variable band than the output ratio of the conventional example, and a large contrast ratio (difference between peak and valley) can be obtained. You can This is because the wavelength can be measured with higher accuracy than the conventional example in a wider wavelength range than the conventional example, and
This means that the oscillation wavelength of the BR-LD 10 can be controlled. Furthermore, the birefringent plate 18 is made of a single material (YVO ₄ ).
Since it is made of a plate, it is possible to easily form a plate having desired characteristics by cutting and polishing. That is, since a precise thin film forming process is not required unlike the dielectric multilayer mirror, the birefringent plate 18 having desired characteristics can be manufactured inexpensively and easily.

The relationship between the output ratio calculated by the output ratio calculating circuit 26 and the laser oscillation wavelength of the DBR-LD 10 is measured in advance, and the corresponding relationship is stored in the control circuit 28 as a look-up table or a function. . Control circuit 28
Applies the output ratio calculated by the output ratio calculation circuit 26 to the table or function to confirm the current wavelength, and controls the reflection wavelength of the DBR of the DBR-LD 10 so that it becomes a desired value. For wavelength measurement applications, the output ratio calculated by the output ratio calculating circuit 26 may simply be applied to the table or function to confirm the current wavelength.

Although YVO ₄ is used as the material of the birefringent plate 18, the present invention is not limited to this material. For example,
Either rutile (TiO ₂ ) crystal or quartz (SiO ₂ ) may be used. Since the rutile crystal has a large amount of birefringence, it can be made thinner. For example, in order to obtain a band equivalent to that shown in FIG. 4, the thickness of the rutile crystal may be 494.6 μm. On the other hand, quartz has a small amount of birefringence, and thus needs to be thick. Similarly, if one wants to obtain a band equivalent to that shown in FIG. 4, the thickness of the crystal must be 12.28 mm. Thicker than YVO ₄ and rutile crystals,
The effect of thickness error due to polishing can be reduced,
Moreover, since the change in refractive index due to temperature is smaller than that of other materials, it is possible to realize a wavelength detector with better temperature stability.

Although it is omitted in the embodiment shown in FIG. 1 for easy understanding, in many cases in practice, the refractive index changes due to temperature change, and the detection wavelength fluctuates due to the refractive index change. There is a problem in measurement accuracy, and eventually in wavelength stabilization accuracy. For that purpose, a compensation mechanism for temperature change may be provided. For example, the refractive index of the birefringent plate 18 at each temperature is measured and stored in advance, and the birefringent plate 18 is
The correspondence relationship between the output ratio and the wavelength may be corrected based on the measurement result of the current temperature. However, this configuration requires a large amount of data and / or complex calculations.

In addition, a medium such as a piezoelectric body whose birefringence amount can be adjusted by an electric field is used as the birefringence plate 18 so that the birefringence amount becomes a desired amount according to the current temperature of the birefringence plate 18. The influence of temperature can be easily eliminated by controlling the applied electric field. FIG. 5 shows a schematic block diagram of the polarization embodiment. The same components as those shown in FIG. 1 are designated by the same reference numerals.

As the birefringent plate 18, a crystal plate 32 and a piezoelectric ceramic plate (PLZT plate or PZT plate) 34 are serially arranged. Crystal plate 32 and piezoelectric ceramic plate 34
Both have birefringence, but the piezoelectric ceramic plate 34
Does not have a sufficient amount of birefringence by itself, the crystal plate 3
Use 2 together. The piezoelectric ceramic plate 34 compensates for the temperature fluctuation of the birefringence of the crystal plate 32. Reference numeral 36 denotes a bias power source that applies a changeable bias voltage to the piezoelectric ceramic plate 34. The temperature sensor 38 detects the ambient temperature, specifically, the temperatures of the crystal plate 32 and the piezoelectric ceramic plate 34. The bias power supply 36 changes the voltage applied to the piezoelectric ceramic plate 34 according to the temperature detected by the temperature sensor 38 so that the birefringence of the piezoelectric ceramic plate 34 compensates for the temperature fluctuation of the birefringence of the crystal plate 32. To do.

By combining a plurality of birefringent plates having different temperature changes, the temperature change of the birefringence can be made zero or extremely small. For example, as shown in FIG. 6, two birefringent plates 40 and 42 are arranged between the plane beam splitter 14 and the polarization prism 20. The birefringent plate 40 is made of alpha barium borate (α-BaB ₂ O ₅ ),
The birefringent plate 42 is made of YVO ₄ . The other components are the same as those of the embodiment shown in FIG. 1 and are given the same reference numerals.

To facilitate understanding, α-BaB ₂ O
_Since the wavelength dispersion of the birefringence of ₅ and YVO ₄ is small, it is neglected, and the change amount of the birefringence with respect to the temperature change is assumed to be constant in the temperature range used, for example, the range of 0 to 100 ° C.

Α-BaB_TwoO₅Birefringence of Δn_BAnd so
The temperature coefficient of is at room temperature, Δn_B= -0.07328 dΔn_B/ dT = -7.3 x 10^-6 And YVO_FourBirefringence of Δn_YAnd its temperature coefficient
At room temperature Δn_Y= 0.2039 dΔn_Y/ dT = -5.5 x 10^-6 Is. The thickness of the birefringent plate 40 is L₁, The thickness of the birefringent plate 42
Only L_TwoAs L₂=-{(DΔn_B/ dT) / (dΔn_Y/ dT)} L₁        (12) To satisfy the relationship_TwoAnd L₁Determine the ratio of. Fruit
When dΔn_B/ dT and dΔn _YSince the sign of / dT is the same, the expression
The absolute value of the right side of (10) is L_TwoAnd the birefringent plate 4
Ordinary ray axis of 0 (α-BaB_TwoO₅Then slow axis) and different
Ordinary ray axis (α-BaB_TwoO₅In the first axis)
Extraordinary ray axis of refraction plate 42 (YVO_FourThen slow axis) and
Ordinary ray axis (YVO_FourThen the first axis)
Thus, the birefringent plates 40 and 42 are arranged.

In this way, the birefringent plates 40 and 42 are
The total amount of birefringence is (ΔnL)_total = Δn_BL₁−Δn_YL₂                  (13) = Δn_BL₁−Δn_Y{(dΔn_B/ dT) / (dΔn_Y/ dT)} L₁ Becomes Subtract the second term from the first term on the right side of equation (13)
That is, as described above, the birefringent plates 40 and 42 are orthogonal to each other.
It is due to the arrangement.

Here, it is assumed that the temperature rises by ΔT.
And the change in total birefringence at that time is (ΔnL)_total ^ΔT = {Δn_B＋ (dΔn_B/ dT) ΔT} L₁ − (Δn_Y＋ (dΔn_Y/ dT) ΔT} L₂ = (ΔnL)_total＋ (dΔn_B/ dT) ΔTL₁ − (DΔn_Y/ dT) ΔT {(dΔn_B/ dT) / (dΔn_Y/ dT)} L₁ = (ΔnL)_total                          (14) Becomes Therefore, the total birefringence of the birefringent plates 40 and 42
The folding amount hardly depends on the temperature.

Total birefringence of birefringent plates 40 and 42
Is set to a desired value, and the birefringent plates 40, 42 are set accordingly.
Thickness L₁, L_TwoShould be decided. For example, L₁33
5 μm, L_TwoIs 442 μm, as shown in FIG.
The wavelength characteristic of the output ratio can be obtained. Wavelength on the horizontal axis, output on the vertical axis
The output ratio calculated by the ratio calculation circuit 26 is shown. At 20 ° C
YVO_FourOnly when used, and a temperature of 0 to 100 ° C
Α-BaB in the range_TwoO ₅And YVO_FourAs shown in FIG.
Almost the same characteristics were obtained as when used for.
The characteristic is shown by a solid line. The broken line is YVO at 80 ° C._Fourof
Shows the characteristics when only using. From FIG. 7 to FIG.
In the example, a stable birefringence amount was obtained with respect to temperature.
I know that

Generally, it is difficult to polish the birefringent plates 18, 40 and 42 to a desired thickness. As a means for finely adjusting the thickness of the birefringent plate, the configuration shown in FIG. 8 can be considered. A trapezoidal birefringent crystal 50 and a wedge-shaped birefringent crystal 52 each having one surface inclined at the same angle are prepared, and the respective inclined surfaces are arranged to face each other. The ordinary ray axis and the extraordinary ray axis of the birefringent crystals 50 and 52 are inclined by 45 ° with respect to the polarization direction of the incident light. The birefringent crystal 52 can be moved by the micrometer 54 in a direction orthogonal to the optical axis 56 (direction parallel to the x axis). The birefringent crystal 52 is constantly pressed against the birefringent crystal 50 by a mechanism (not shown). The birefringent crystals 50 and 52 become a birefringent plate having parallel planes as a whole, and the birefringent crystal 52 is adjusted by the micrometer 54.
By moving in the direction orthogonal to the optical axis 56, the thickness can be finely adjusted.

[0056]

As can be easily understood from the above description, according to the present invention, it is possible to measure the wavelength of the light source with high precision and in a wide wavelength range with a cheap and simple structure. Therefore, the wavelength of the light source can be controlled and stabilized with a simple and inexpensive structure.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a polarization direction of input light of a birefringent plate 18, an optical axis of a birefringent medium 18 and an optical axis of a polarizing prism 20.

FIG. 3 is a diagram showing the relationship between the polarization phase difference Δφ of the birefringent plate 18 and the wavelength.

FIG. 4 is a diagram showing a relationship between an output ratio and a wavelength in this example and a conventional example.

FIG. 5 is a schematic configuration block diagram of an embodiment having a temperature compensation function by a piezoelectric ceramic plate.

FIG. 6 is a schematic block diagram of an embodiment in which a plurality of birefringent plates are combined.

FIG. 7 is a characteristic diagram of the embodiment shown in FIG.

FIG. 8 is a perspective view of a mechanism for adjusting the thickness of the birefringent plates 18, 40 and 42.

FIG. 9 is a diagram showing transmission / reflection characteristics and output ratio of a Fabry-Perot etalon.

[Explanation of symbols]

10: DBR-LD 12: Collimator lens 14: Planar beam splitter 16: Photodiode 18: Birefringent plate 20: Polarizing prism 22, 24: Photodiode (PD) 26: Output ratio calculation circuit 28: Control circuit 30: Look-up table 32: Crystal plate 34: Piezoelectric ceramic plate 36: Bias power supply 38: Temperature sensor 40, 42: birefringent plate 50, 52: birefringent crystal 54: Micrometer 56: Optical axis

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01S 3/13 H01S 3/13 5/125 5/125 F term (reference) 2G065 AB09 AB10 BA09 BB31 2H042 CA09 CA14 CA17 2H049 BA05 BA06 BA42 BB03 BC21 5F072 AB13 HH02 JJ05 KK15 YY15 5F073 AA63 AA64 AA65 AB15 AB30 BA02 EA29 GA02 GA12

Claims (25)

[Claims] 1. A wavelength tunable light source composed of a resonator having a reflector whose reflection wavelength can be changed and a laser medium arranged in the resonator, and output light of the wavelength tunable light source is input to the output light of the output light. A birefringent plate having an optical axis inclined at a predetermined angle with respect to the polarization direction, a polarization separator that separates the output light of the birefringent plate into two polarization directions orthogonal to each other, and one of the polarization separators A first light receiving element that receives the polarized light component of the second light receiving element, a second light receiving element that receives the other polarized light component separated by the polarization separator, and a predetermined output from the outputs of the first and second light receiving elements. A wavelength stabilized light source, comprising: an output ratio calculation circuit for calculating a ratio; and a control circuit for controlling the reflection wavelength of the reflector to a predetermined value according to the calculation result of the output ratio calculation circuit. 2. The wavelength stabilized light source according to claim 1, wherein the predetermined angle is 45 °. 3. The polarization separator separates the output light of the birefringent plate into a polarization component in the same direction as the polarization direction of the output light of the wavelength tunable light source and a polarization component orthogonal thereto. Or the wavelength-stabilized light source described in 2. 4. A temperature sensor is further provided, and the birefringent plate comprises a first birefringent material and a second birefringent material whose birefringence amount is adjusted according to the temperature detected by the temperature sensor. Item 1. A wavelength-stabilized light source according to item 1. 5. The wavelength stabilized light source according to claim 4, wherein the second birefringent material is a piezoelectric ceramic material. 6. A second birefringence in which the birefringent plate comprises a first birefringent material and a birefringence amount of a temperature change that cancels a temperature change of a birefringence amount of the first birefringent material. The wavelength-stabilized light source according to claim 1, comprising a material. 7. The birefringent plate is composed of a first birefringent material and a second birefringent material showing a change in birefringence amount having the same sign with respect to a temperature change. 7. The wavelength stabilization according to claim 6, wherein the first and second birefringent materials are arranged so that the ordinary ray axis and the extraordinary ray axis coincide with the extraordinary ray axis and the ordinary ray axis of the second birefringent material. Light source. 8. When the output of the first light receiving element is P _x and the output of the second light receiving element is P _y , the output ratio is (P _x −P _y ).
The wavelength stabilized light source according to claim 1, wherein the wavelength stabilized light source is / (P _x + P _y ). 9. A third light receiving element for detecting the output power of the wavelength tunable light source is further provided, and the control circuit controls the output power of the wavelength tunable light source according to the output of the third light receiving element. The wavelength stabilized light source according to claim 1. 10. A wavelength control device for controlling an output wavelength of a wavelength tunable light source, comprising: a wavelength control device that receives an output light of the wavelength tunable light source and has an optical axis inclined at a predetermined angle with respect to a polarization direction of the output light. A refraction plate, a polarization separator for separating output light of the birefringence plate into two polarization directions orthogonal to each other, a first light receiving element for receiving one polarization component separated by the polarization separator, A second light receiving element that receives the other polarization component separated by the polarization separator, an output ratio calculation circuit that calculates a predetermined output ratio from the outputs of the first and second light receiving elements, and the output ratio calculation A wavelength control device, comprising: a control circuit that controls the reflection wavelength of the reflector to a predetermined value according to the calculation result of the circuit. 11. The predetermined angle is 45 °.
The wavelength control device according to 0. 12. The polarization splitter separates the output light of the birefringent plate into a polarization component in the same direction as the polarization direction of the output light of the wavelength tunable light source and a polarization component orthogonal thereto. Alternatively, the wavelength control device according to item 11. 13. A temperature sensor is further provided, and the birefringent plate comprises a first birefringent material and a second birefringent material whose birefringence amount is adjusted according to the temperature detected by the temperature sensor. Item 11. The wavelength control device according to item 10. 14. The wavelength control device according to claim 13, wherein the second birefringent material is a piezoelectric ceramic material. 15. The birefringent plate comprises a first birefringent material,
The wavelength control device according to claim 10, comprising a second birefringent material having a temperature change birefringence amount that cancels a temperature change of the birefringence amount of the first birefringent material. 16. The birefringent plate is composed of a first birefringent material and a second birefringent material showing a change in birefringence amount having the same sign with respect to a temperature change. 16. The wavelength control according to claim 15, wherein the first and second birefringent materials are arranged so that the ordinary ray axis and the extraordinary ray axis coincide with the extraordinary ray axis and the ordinary ray axis of the second birefringent material. apparatus. 17. When the output of the first light receiving element is P _x and the output of the second light receiving element is P _y , the output ratio is (P _x −
The wavelength control device according to claim 10, wherein P _y ) / (P _x + P _y ). 18. A wavelength measuring device for measuring a wavelength of a light source, comprising: a birefringent plate having an optical axis to which output light of the light source is input and which is inclined at a predetermined angle with respect to a polarization direction of the output light. A polarization separator for separating the output light of the birefringent plate into two polarization directions orthogonal to each other, a first light receiving element for receiving one polarization component separated by the polarization separator, and a separation for the polarization separator. The second light receiving element that receives the other polarized light component, the output ratio calculation circuit that calculates a predetermined output ratio from the outputs of the first and second light receiving elements, and the calculation result of the output ratio calculation circuit A wavelength measuring device, comprising: a matching circuit for matching the relationship between the output ratio and the wavelength, which is measured in advance. 19. The predetermined angle is 45 °.
8. The wavelength measuring device according to 8. 20. The polarization splitter separates the output light from the birefringent plate into a polarization component in the same direction as the polarization direction of the output light from the light source and a polarization component orthogonal to the polarization component. The wavelength measuring device described in. 21. A temperature sensor is further provided, and the birefringent plate comprises a first birefringent material and a second birefringent material whose birefringence amount is adjusted according to the temperature detected by the temperature sensor. Item 18. The wavelength measuring device according to item 18. 22. The wavelength measuring device according to claim 21, wherein the second birefringent material is a piezoelectric ceramic material. 23. The birefringent plate comprises a first birefringent material,
19. The wavelength measuring device according to claim 18, comprising a second birefringent material having a temperature change birefringence amount that cancels a temperature change of the birefringence amount of the first birefringence material. 24. The birefringent plate is composed of a first birefringent material and a second birefringent material showing a change in birefringence amount having the same sign with respect to a temperature change. 24. The wavelength measurement according to claim 23, wherein the first and second birefringent materials are arranged so that the ordinary ray axis and the extraordinary ray axis coincide with the extraordinary ray axis and the ordinary ray axis of the second birefringent material. apparatus. 25. When the output of the first light receiving element is P _x and the output of the second light receiving element is P _y , the output ratio is (P _x −
19. The wavelength measuring device according to claim 18, wherein P _y ) / (P _x + P _y ).