JP2015154052A - Wavelength filter and laser - Google Patents

Abstract

To increase a mode gain difference. SOLUTION: The light output from one end of an optical amplifier 30 having a gain band is arranged in cascade in a path 55 through which the light propagates so that the light is input to the one end. A plurality of ring resonators 20a, 20b having a highest first peak in the gain band, a second peak adjacent to the first peak, and a third peak adjacent to the second peak; and A transmission unit 22 arranged in cascade with the plurality of ring resonators, wherein the transmittance at the wavelength of the second peak is lower than the transmittance at the wavelengths of the first peak and the third peak, and transmits the light; Wavelength filter provided. [Selection] Figure 4

Description

  The present invention relates to a wavelength filter and a laser, for example, a wavelength filter and a laser having a ring resonator.

  There is digital coherent optical communication as a large-capacity optical communication method using optical phase information. In digital coherent optical communication, large-capacity communication is possible by including phase modulation, such as DP-QPSK (Dual-Polarization Duadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation). Such a light source for digital coherent communication is required to have a narrow laser oscillation line width. In particular, as the capacity increases like 16QAM, a narrower laser oscillation line width is required. Patent Document 1 describes a laser using a ring resonator. Non-Patent Document 1 describes a laser in which two ring resonators are connected in cascade as a wavelength filter. Non-Patent Document 2 describes a laser using a ring resonator and a Mach-Zehnder interferometer.

JP 2011-164406 A

Appl. Phys. Express Vol. 5, 082701 (2012) IEEE J. Quantum Electron. Vol. 15, 488 (2009)

  The laser described in Non-Patent Document 1 can narrow the laser resonance line width. However, the mode gain difference is small.

  The present invention has been made in view of the above problems, and an object thereof is to increase a mode gain difference.

  In the present invention, light output from one end of an optical amplifier having a gain band is arranged in cascade in a path through which the light propagates so that the light is input to the one end, the frequency intervals are different from each other, and the total transmittance is A plurality of ring resonators having a highest first peak in the gain band, a second peak adjacent to the first peak, and a third peak adjacent to the second peak; and the plurality of ring resonances in the path A transmission unit that is arranged in cascade with a vessel and has a transmittance at the wavelength of the second peak lower than the transmittance at the wavelength of the first peak and the third peak, and transmits the light. This is a wavelength filter.

  The said structure WHEREIN: The said permeation | transmission part can be set as the structure which is an asymmetric Mach-Zehnder interferometer which has a frequency space | interval which is 1.5 times or more and 3 times or less of the average value of the frequency interval of these ring resonators.

  In the above configuration, the path is branched at a branch point, the branched path is a loop path, and the plurality of ring resonators and the transmission unit are arranged in cascade in the loop path. Can do.

  The said structure WHEREIN: It can be set as the structure which comprises the wavelength change part which changes the wavelength of a said 1st peak by changing at least 1 resonance wavelength of these ring resonators.

  The said structure WHEREIN: The said wavelength change part can be set as the structure which changes the temperature of at least 1 of these ring resonators.

  The said structure WHEREIN: The said path | route can be set as the structure which consists of an optical waveguide formed on the same board | substrate.

  The said structure WHEREIN: The optical waveguide formed on the said same board | substrate can be set as the structure which has a core which consists of silicon | silicone, and a clad which consists of silicon oxide.

  According to the present invention, an optical coupler for branching a first optical waveguide coupled to an optical amplifier having a gain band into a pair of second optical waveguides and a pair of second optical waveguides are coupled to each other, and the frequency interval is A pair of ring resonators having a first peak having a highest total transmittance in the gain band and a pair of third waveguides respectively coupled to the pair of ring resonators, An asymmetric Mach-Zehnder interferometer having a frequency interval that is coupled to each other, a transmittance peak overlaps with the first peak, and has a frequency interval that is 1.5 to 3 times the average value of the frequency interval of the pair of ring resonators; And a wavelength filter characterized by comprising:

  In the above configuration, the output power of the laser light emitted from the optical amplifier may be 40 mW or more, and the light intensity in the ring resonator may be 40 mW or less.

  The present invention is a laser comprising the optical amplifier and the wavelength filter.

  According to the present invention, the mode gain difference can be increased.

FIGS. 1A and 1B are schematic views showing lasers according to comparative examples 1 and 2, respectively. FIG. 2A and FIG. 2B are schematic diagrams showing the transmittance with respect to the wavelength in Comparative Examples 1 and 2. FIG. FIGS. 3A and 3B are diagrams showing fiber coupling power with respect to the wavelength in Comparative Example 1. FIG. FIG. 4 is a schematic diagram of a laser according to the first embodiment. FIG. 5A and FIG. 5B are diagrams showing the transmittance with respect to the wavelength in the first embodiment. 6A is a plan view of the wavelength filter in the first embodiment, FIG. 6B is a sectional view of a part of the wavelength filter, FIG. 6C is an enlarged view of the ring resonator, and FIG. d) is an enlarged view of the vicinity of the ring resonator and the optical waveguide. FIG. 7 is a block diagram showing a laser measurement system. FIG. 8A is a diagram showing the output power of the laser beam with respect to the SOA injection current in Example 1, and FIG. 8B is a diagram showing the fiber coupling power with respect to the wavelength. FIG. 9 is a diagram showing the wavelength with respect to the heater power. FIG. 10 is a diagram illustrating the fiber coupling power with respect to the wavelength in Example 1, and a diagram illustrating the SMSR with respect to the wavelength. FIG. 11A is a diagram showing the power with respect to the RF frequency, and FIG. 11B is a diagram showing the line width with respect to the output power. FIG. 12 is a diagram showing the maximum output power P _max with respect to the light intensity P _ring in the ring resonator in Comparative Examples 1 and 2 and Example 1.

  First, a comparative example will be described. FIGS. 1A and 1B are schematic views showing lasers according to comparative examples 1 and 2, respectively. As shown in FIGS. 1A and 1B, the lasers according to Comparative Examples 1 and 2 include a wavelength filter 10 and an optical amplifier 30. The light 50 output from one end of the optical amplifier 30 propagates through the path 55 in the wavelength filter 10 and returns to the one end of the optical amplifier 30 to be input. The path 55 transmits light having a specific wavelength and blocks light having other wavelengths. A reflection plate 32 is disposed at the other end of the optical amplifier 30. The reflector 32 transmits part of the light having a specific wavelength and reflects the rest. As a result, light having a specific wavelength transmitted through the path 55 is amplified, and stimulated emission occurs in the optical amplifier 30. As a result, a laser beam 60 having a specific wavelength is output from the other end of the optical amplifier 30.

  As shown in FIG. 1A, in the laser 110 according to Comparative Example 1, the path 55 through which light propagates includes a path 54 and a path 56. The route 54 branches to a route 56 at a branch point 58. In the path 54, optical waveguides 12, 18, and 19 and ring resonators 20a and 20b are arranged. The optical waveguide 12 is optically coupled to the optical amplifier 30. The ring resonator 20 a is optically coupled to the optical waveguide 12. The optical waveguide 18 is optically coupled to the ring resonator 20a. The ring resonator 20 b is optically coupled to the optical waveguide 18. The optical waveguide 19 is optically coupled to the ring resonator 20b. One end of the optical waveguide 19 is joined to the loop-shaped optical waveguide 17. The optical waveguide 17 corresponds to the path 56.

  The light 50 output from one end of the optical amplifier 30 sequentially passes through the ring resonators 20 a and 20 b in the path 54. The light 50 is turned back along the path 56, sequentially passes through the ring resonators 20 b and 20 a in the path 54, and is input to the one end of the optical amplifier 30 as the light 52. When viewed from the light 50, the plurality of ring resonators 20 a and 20 b are arranged in cascade in the path 54.

  As shown in FIG. 1B, in the laser 112 according to Comparative Example 2, the optical waveguide 12 corresponds to the path 54. The optical coupler 26 corresponds to the branch point 58. In the path 56, 14a, 14b and 16 and ring resonators 20a and 20b are arranged. The optical waveguide 12 is coupled to the optical amplifier 30. The optical coupler 26 branches the optical waveguide 12 into a pair of optical waveguides 14a and 14b. A pair of ring resonators 20a and 20b are optically coupled to the optical waveguides 14a and 14b, respectively. The pair of optical waveguides 16 are optically coupled to the ring resonators 20a and 20b.

  The light 50 output from one end of the optical amplifier 30 propagates through the path 54 and branches into a plurality of lights 50 a and 50 b at the branch point 58. The light 50a propagates in the counterclockwise direction through the path 56, thereby passing through the ring resonators 20a and 20b in order. The light 50b propagates in the clockwise direction through the path 56 to pass through the ring resonators 20b and 20a in order. Lights 50 a and 50 b are combined at the branch point 58, propagated through the path 54, and input as light 52 to one end of the optical amplifier 30. When viewed from the light beams 50 a and 50 b branched at the branch point 58, the plurality of ring resonators 20 a and 20 b are arranged in cascade in the path 56.

  In Comparative Example 1, light 50 passes through ring resonators 20a and 20b twice, whereas in Comparative Example 2, light 50 passes through ring resonators 20a and 20b only once. Thereby, the resonator length of the comparative example 2 can be shortened compared with the comparative example 1. If the light intensity in the ring resonators 20a and 20b increases when the laser beam 60 is output at high power, it is considered that resonance in the submode occurs due to the nonlinear optical effect, and laser oscillation becomes unstable. As in Comparative Example 2, if the number of times the light 50 passes through the ring resonators 20a and 20b is small, the light intensity in the ring resonators 20a and 20b can be suppressed to a low level even at high output. It is thought that there is not. Therefore, in Comparative Example 2, there is a possibility that instability of laser oscillation at high output can be suppressed.

  FIG. 2A and FIG. 2B are schematic diagrams showing the transmittance with respect to the wavelength in Comparative Examples 1 and 2. FIG. Referring to FIG. 2A, the optical amplifier 30 amplifies light having a wavelength in the gain band B. The broken line indicates the transmittance R1 of the ring resonator 20a, and the dotted line indicates the transmittance R2 of the ring resonator 20b. The solid line indicates the total transmittance R1 + R2 of the ring resonators 20a and 20b. Since the ring resonators 20a and 20b are arranged in a column in the path 55, the transmittance of the light propagated through the path 55 is R1 + R2.

  Each of the ring resonators 20a and 20b resonates at the resonance wavelength, and the transmittance is maximized. The transmittance is minimum at wavelengths in the middle of the resonance wavelength. The interval between the resonant wavelengths is constant, and this interval is called a frequency interval (FSR: Free Spectral Range). The frequency intervals of the ring resonators 20a and 20b are FSR1 and FSR2, respectively. The frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b are different from each other. The frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b can be set by the circumference of the ring resonators 20a and 20b.

  Since the frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b are different, due to the vernier effect, the total transmittance R1 + R2 has the highest transmittance at the wavelength at which the resonance wavelengths of the ring resonators 20a and 20b overlap most ( Mode M1) is entered. A mode M2 peak is formed adjacent to both sides of the mode M1 peak, and a mode M3 peak is formed adjacent to both sides of the mode M3 peak. Mode M2 has a smaller overlap of resonance wavelengths than mode M1. For this reason, the peak of mode M2 is lower than that of mode M1. Similarly, the peak of mode M3 is lower than that of mode M2. The interval of the peak wavelength in mode M1 is defined as an interval Δλ1. An interval between the peak wavelengths of the modes M1 and M2 and an interval between the peak wavelengths of M2 and M3 are defined as an interval Δλ2. A transmittance difference ΔT between the modes M1 and M2. The transmittance difference ΔT corresponds to a mode gain ratio (SMSR: Side-Mode Suppression Ratio) of laser light. By making the interval Δλ1 substantially the same as or larger than the gain band B, only one mode M1 is formed in the gain band B.

  With reference to FIG. 2B, the vicinity of the modes M1 and M2 is enlarged. A resonance mode FP of an FP (Fabry-Perot) longitudinal mode is formed in the total transmittance R1 + R2. The frequency interval FSR3 in the FP longitudinal mode decreases as the resonator length of the wavelength filter 10 increases, and increases as the resonator length decreases.

  As in Comparative Example 1, a plurality of ring resonators 20a and 20b having different frequency intervals FSR1 and FSR2 are cascade-connected in the path 55 to form the wavelength filter 10. The ring resonators 20a and 20b have a high Q value. For this reason, by using the ring resonators 20a and 20b in the wavelength filter 10, the laser can output the laser beam 60 having a narrow wavelength line width. However, in Comparative Example 1, when the injection current of the optical amplifier 30 is increased, the laser oscillation becomes unstable and the output power of the laser beam 60 is limited.

Lasers according to Comparative Examples 1 and 2 were produced. The core material of the optical waveguide and the ring resonator is silicon (Si), and the clad material is silicon oxide (SiO ₂ ). The optical amplifier 30 is a semiconductor optical amplifier (SOA) using InP. The resonator lengths of the silicon optical waveguides in the wavelength filters 10 of Comparative Examples 1 and 2 are 1.7 mm and 1.0 mm, respectively. The laser oscillation wavelength is L band (1570 nm to 1612 nm). Others are the same as the laser produced in Example 1 mentioned later.

  FIGS. 3A and 3B are diagrams showing fiber coupling power with respect to the wavelength in Comparative Example 1. FIG. The fiber coupling power is a power measured by coupling a part of the laser beam 60 to the fiber. As shown in FIG. 3A, when the injection current of the SOA which is the optical amplifier 30 is increased, oscillations 62 corresponding to a plurality of FP longitudinal modes are observed. Further, as shown in FIG. 3B, a plurality of resonance modes 64 due to the vernier effect are observed. Such a multimode oscillation of a plurality of modes limits the maximum output power of the laser beam.

  As described above, in Comparative Example 1, the cause of the occurrence of multimode oscillation is not clear. However, in Comparative Example 1, the resonator length is long, the FP longitudinal mode interval FSR3 is small, and / or the ring resonator 20a. It is thought that this is because the light intensity within 20b is high.

In Comparative Example 2, the distance that the light reciprocates in the optical waveguide is short, so that the resonator length can be shortened. Further, the light intensity in the ring resonators 20a and 20b can be weakened. Thereby, multimode oscillation is suppressed and the maximum output power can be increased. Table 1 is a table showing the FP longitudinal mode interval FSR3, the maximum output power, and the SMSR in Comparative Examples 1 and 2.

  As shown in Table 1, the FP longitudinal mode interval FSR3 of Comparative Example 2 is 123 pm, which is larger than 94 pm of Comparative Example 1. The maximum output power of Comparative Example 2 is 34.9 mW, which is larger than 20 mW of Comparative Example 1. Thus, by increasing the FSR3 in the FP longitudinal mode, multimode oscillation can be suppressed and the maximum output power can be increased. However, the SMSR is about 30 dB in both Comparative Examples 1 and 2, and is not so large.

  Thus, according to Comparative Example 2, the maximum output power can be increased. However, the SMSR is small. If the SMSR is small, it may cause multimode oscillation. If the SMSR can be increased, the maximum output power may be increased. An embodiment that can increase the SMSR will be described below.

  FIG. 4 is a schematic diagram of a laser according to the first embodiment. As shown in FIG. 4, the laser 100 includes a wavelength filter 10 and an optical amplifier 30. An asymmetric Mach-Zehnder interferometer (AMZI) 22 is arranged in the path 56 in cascade with a plurality of ring resonators 20a and 20b. That is, when viewed from the light beams 50 a and 50 b branched at the branch point 58, the plurality of ring resonators 20 a and 20 b and the AMZI 22 are arranged in cascade in the path 56. Optical waveguides 16a and 16b are optically coupled to ring resonators 20a and 20b, respectively. Both ends of the AMZI 22 are optically coupled to one ends of the optical waveguides 16a and 16b, respectively. The AMZI 22 has a plurality of optical waveguides 24a and 24b. The path lengths of the optical waveguides 24a and 24b are different from each other, and light propagating through the paths 54a and 54b is modulated by the AMZI 22. Other configurations are the same as those of the comparative example 2, and the description thereof is omitted.

  FIG. 5A and FIG. 5B are diagrams showing the transmittance with respect to the wavelength in the first embodiment. FIG. 5A is a diagram showing the transmittances R1, R2 and AMZI of the ring resonators 20a and 20b and the AMZI 22. FIG. 5B is a diagram showing the total transmittance R1 + R2 of the ring resonators 20a and 20b. It is a figure which shows the sum total R1 + R2 + AMZI of the transmittance | permeability of resonator 20a, 20b and AMZI22. Ring resonators 20 a, 20 b and AMZI 22 are arranged in tandem in path 55. For this reason, the transmittance of the light propagated through the path 55 is R1 + R2 + AMZI.

  As shown in FIG. 5A, the transmittance AMZI of the AMZI 22 is set to be maximum at a wavelength corresponding to the mode M1, and to be minimum at a wavelength corresponding to the adjacent mode M2. That is, the path difference between the optical waveguides 24a and 24b is set so that the frequency interval FSR4 of the AMZI 22 is approximately twice the frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b.

  As shown in FIG. 5B, the peak of R1 + R2 in modes M1 and M3 and the peak of transmittance AMZI of AMZI22 overlap. For this reason, the total transmittance R1 + R2 + AMZI of the modes M1 and M3 is almost R1 + R2. On the other hand, in mode M2, since the transmittance of AMZI22 is low, the total transmittance R1 + R2 + AMZI is much smaller than R1 + R2. As a result, the SMSR becomes the difference between the peaks of the modes M1 and M3, and the SMSR can be increased.

  A laser according to Example 1 was manufactured. 6A is a plan view of the wavelength filter in the first embodiment, FIG. 6B is a sectional view of a part of the wavelength filter, FIG. 6C is an enlarged view of the ring resonator, and FIG. d) is an enlarged view of the vicinity of the ring resonator and the optical waveguide.

  As shown in FIG. 6B, a silicon oxide film 42 is formed on the silicon substrate 40. Silicon fine wires 44 are formed in the silicon oxide film 42. The width of the silicon fine wire 44 is W and the height is H. The silicon thin wire 44 functions as the core of the optical waveguide, and the silicon oxide film 42 functions as the cladding. A metal film 46 is formed on the silicon oxide film 42 above the silicon thin wire 44. The metal film 46 is a Ti film and a Pt film from the silicon oxide film 42 side.

  As shown in FIG. 6A, the optical waveguides 12, 14a, 14b, 16a, and 16b, the ring resonators 20a and 20b, and the AMZI 22 are formed by an optical waveguide having a silicon thin wire 44 as a core. The metal film 46 forms heaters 34a to 34c and pads 48. The heaters 34a to 34c are respectively formed on one of the ring resonators 20a and 20b and one optical waveguide 24b of the AMZI 22. The pad 48 is used to flow current from the outside to the heaters 34a to 34c. One end of the optical waveguide 12 is butt-joined to the OSA via an SSC (Spot Size Converter) 45. A refractive index matching gel having a refractive index of 1.46 was used for butt joint bonding.

  As shown in FIG. 6C, the ring resonator 20b is disposed adjacent to the bus optical waveguides 14b and 16b. The ring resonator 20b includes an arc region 36a and a straight region 36b. The arc region 36a has a radius r of curvature. By changing the distance of the straight region 36b, the circumference of the ring resonator 20b can be changed. As shown in FIG. 6D, the bus optical waveguides 14 and 16 and the ring resonator 20 are separated so as to have a gap G1. The distance that the bus optical waveguides 14 and 16 and the ring resonator 20 run in parallel is the coupler length L1. The coupling efficiency between the bus optical waveguides 14 and 16 and the ring resonator 20 can be adjusted by appropriately setting the gap G1 and the coupler length L1.

Hereafter, each dimension etc. of the laser which concerns on the produced Example 1 are shown below.
Footprint size: 2.6 x 0.5mm ²
The footprint is a substrate on which the wavelength filter and the SOA are mounted.
Silicon wire 44 width W: 400 nm
The height H of the thin silicon wire 42; 220 nm
Resonator length of wavelength filter 10: 1.0 mm
SOA resonator length: 1.3mm
Total resonance length of wavelength filter and SOA: 2.3 mm
AMZI FSR4: 1600GHz
AMZI path difference: 42.8μm

Ring resonator radius r: 12 μm
Coupling efficiency between bus and ring: 0.156
Gap G1: 300nm between bus and ring
Bus length and ring length L1: 2.75 μm
Ring resonator 20a FSR1: 727GHz
FSR2 of ring resonator 20b: 800 GHz
Circumference length of ring resonator 20a: 85.5 μm
Circumference length of ring resonator 20b: 94.0 μm
Ti film / Pt film thickness of metal film 46: 10 nm / 100 nm
The width of the metal film 46 in the heater: 8 μm
Heater resistance: about 200Ω

  FIG. 7 is a block diagram showing a laser measurement system. As shown in FIG. 7, the measurement system includes a spherical fiber 70, an isolator 71, optical couplers 72 and 73, an optical spectrum analyzer 74, a power meter 75, a line width measurement system 76, an SOA control unit 77, and a heater control unit. 78. The tip sphere fiber 70 condenses the laser light emitted from the laser 100. The isolator 71 isolates the collected light. The optical couplers 72 and 23 branch light. The optical spectrum analyzer 74 measures the spectrum of light. The power meter 75 measures the power of light. The line width measurement system 76 measures the line width of the laser light. The SOA control unit 77 keeps the temperature of the SOA, which is the optical amplifier 30, constant using TEC (thermoelectric coolers). In the following measurements, the SOA temperature was set to 25 ° C. The SOA control unit 77 controls the injection current of the SOA. The heater control unit 78 controls the current applied to the heaters 34 b and 34 c via the pad 48.

  FIG. 8A is a diagram showing the output power of the laser beam with respect to the SOA injection current in Example 1, and FIG. 8B is a diagram showing the fiber coupling power with respect to the wavelength. FIG. 8A shows the result of measuring output power using a power meter 75 when the wavelength is 1590 nm. FIG. 8B shows the result of measuring the optical spectrum using the optical spectrum analyzer 74 when the SOA injection current is 300 mA.

  As shown in FIG. 8A, the output power of Example 1 is larger than that of Comparative Example 1. The maximum output power was 42.2 mW when the SOA injection current was 300 mA. The threshold current is 23 mA. The sleep efficiency corresponding to the inclination is 0.151 mW / mA.

  As shown in FIG. 8B, at the maximum output power, adjacent FP longitudinal mode oscillation does not occur, and multimode oscillation as in FIG. 3A of Comparative Example 1 does not occur. The gain difference with respect to other modes of the oscillating main mode is as large as 49.8 dB. Thus, in the first embodiment, multimode oscillation can be suppressed and the output power can be increased as compared with the first comparative example. The maximum output power can be 16 dBm or more required in 16QAM.

  Next, the wavelength of the laser beam is changed using a heater. FIG. 9 is a diagram showing the wavelength with respect to the heater power. A current is passed through the heater 34a corresponding to the ring resonator 20a, and no current is passed through the heater 34b corresponding to the ring resonator 20b. As shown in FIG. 9, when the power of the heater 34a is increased, the temperature of the silicon fine wire 44 of the ring resonator 20a increases, and the refractive index of the silicon fine wire 44 increases. Thereby, the resonance wavelength of the ring resonator 20a becomes longer, and the wavelength of the mode M1 becomes longer. Therefore, the laser oscillation wavelength becomes long. Since the mode M1 shifts to the high wavelength side, the peak of the transmittance of the AMZI 22 in FIG. 5A is changed corresponding to the wavelength of the mode M1. A white circle in FIG. 9 indicates that power of 111.7 mW is applied to the heater 34c of the AMZI 22, and a white triangle indicates that power is not applied to the heater 34c. Thus, by controlling the path difference of the AMZI 22 using the heater 34c, the transmittance peak of the AMZI 22 can be shifted corresponding to the wavelength shift of the mode M1.

  FIG. 10 is a diagram illustrating the fiber coupling power with respect to the wavelength in Example 1, and a diagram illustrating the SMSR with respect to the wavelength. The spectrum and SMSR of each point of FIG. 9 are shown. As shown in FIG. 10, one mode oscillates at any wavelength. The SMSR is about 40 dB and a minimum of 38 dB.

  Thus, the laser oscillation wavelength can be made variable by adjusting the temperatures of the ring resonator 10a and the AMZI 22 with the heaters 34a and 34c. 9 and 10, the laser oscillation wavelength can be made variable in the range of 1.570 μm to 1.610 μm which is the L band. By making the oscillation wavelength of the laser light variable, it is possible to apply to a wavelength multiplexing system.

  The line width of the resonance mode was measured by the delayed self-heterodyne method using the line width measurement system 76. FIG. 11A is a diagram showing the power with respect to the RF frequency, and FIG. 11B is a diagram showing the line width with respect to the output power. With reference to Fig.11 (a), the measurement result in Example 1 and the fitting line which assumed Lorentz distribution are shown. The output power is 42.2 mW. From FIG. 11A, the line width is about 83 kHz.

  With reference to FIG.11 (b), the line width measurement result of the comparative example 1 and Example 1 is shown with a white circle and a white triangle. The line width calculated using the method described in IEEE J. Quantum Electron., Vol QE-18, No. 2 (1982) is shown by a curve. In Example 1, the line width is larger than that in Comparative Example 1, but when the output power is 10 mW or more, the line width is 100 kHz or less required for 16QAM. The reason why the line width of Example 1 is larger than that of Comparative Example 1 is that the resonator length of Example 1 is short.

FIG. 12 is a diagram showing the maximum output power P _max with respect to the light intensity P _ring in the ring resonator in Comparative Examples 1 and 2 and Example 1. In FIG. 12, in Comparative Example 1, two lasers having coupling coefficients of 0.15 and 0.22 between the ring resonators 20a and 20b and the bus optical waveguides 12, 18, and 19 were produced. The light intensity in the ring resonators 20a and 20b is a simulation result.

  As shown in FIG. 12, when the light intensity in the ring resonators 20a and 20b exceeds 40 mW, the maximum output power decreases. This is presumably because when the light intensity in the ring resonators 20a and 20b increases, resonance in the submode occurs due to the nonlinear optical effect, and laser oscillation at the time of high output of the laser light becomes unstable. Thus, the light intensity in the ring resonators 20a and 20b is preferably 40 mW or less.

  In Comparative Example 1, when the coupling coefficient is 0.22, the light intensity becomes weak and the maximum output power becomes large. However, the difference in transmittance between modes is reduced, and the mode gain difference is reduced to prevent single mode oscillation. As described above, in Comparative Example 1, when the mode gain difference is increased, the light intensity in the ring resonators 20a and 20b is increased, and the laser oscillation becomes unstable. On the other hand, in Comparative Example 2 and Example 1, since the light 50 passes through the ring resonators 20a and 20b only once, the light intensity in the ring resonators 20a and 20b can be reduced. Therefore, laser oscillation can be stabilized.

  As shown in Table 1, in Example 1 provided with AMZI 22, even when the resonator length and FSR3 are the same as those in Comparative Example 2, adjacent modes can be suppressed as shown in FIG. As a result, the SMSR can be improved from 30 dB in Comparative Example 2 to 38 dB in Example 1. Thereby, it is stable even if the output power is increased. For this reason, the maximum output power can be increased from 34.9 mW in Comparative Example 1 to 42.2 mW. Further, by providing a heater, the wavelength can be varied within the L band. The wavelength variable width is 61.7 nm. The first embodiment can be applied to other bands such as a C band (wavelength is 1.525 μm to 1.570 μm) by appropriately designing the SOA and the wavelength filter. As described above, the manufactured laser according to Example 1 has an output power of 40 mW (16 dBm) or more and can have a line width of 100 kHz or less. Therefore, it is used as a light source for 16QAM digital coherent communication. it can.

  As described above, according to the first embodiment, the light 50 propagates in the path 55 so that the light 50 output from one end of the optical amplifier 30 returns to the one end of the optical amplifier 30 and is input as shown in FIG. The plurality of ring resonators 20 a and 20 b are arranged in cascade in the path 55. The AMZI 22 (transmission part) is arranged in cascade with the plurality of ring resonators 20 a and 20 b in the path 55.

  In such a configuration, as shown in FIGS. 5A and 5B, the total transmittance R1 + R2 of the plurality of ring resonators 20a and 20b is the peak of the highest mode M1 in the gain band B. (First peak), mode M2 peak (second peak), and mode M3 peak (third peak). In the AMZI 22, the transmittance at the peak wavelength of the mode M2 is lower than the transmittance at the peak wavelengths of the modes M1 and M3. Thereby, the transmittance | permeability in mode M2 of the light which propagates the path | route 55 can be suppressed. Therefore, laser oscillation due to mode M2 can be suppressed. Thereby, the mode gain difference can be increased.

  In order to further increase the mode gain difference, the transmittance at the peak wavelength of the mode M2 is preferably less than or equal to ½ of the transmittance at the peak wavelengths of the modes M1 and M3, more preferably 1/10 or less. preferable.

  In the first embodiment, the example in which the AMZI 22 is arranged in the comparative example 2 has been described. However, the AMZI 22 may be arranged in cascade in the ring resonators 20a and 20b in the path 54 of the comparative example 1, for example. Further, the AMZI 22 can be arbitrarily arranged in the path 55 as long as it is arranged in cascade in the plurality of ring resonators 20a and 20b. Further, the number of ring resonators 20a and 20b may be three or more.

  In the first embodiment, the plurality of ring resonators 20 a and 20 b and the AMZI 22 are arranged in cascade in the loop path 56. Thereby, the first embodiment can reduce the resonator length. Therefore, the mode gain difference can be increased and the frequency interval FSR3 of the FP longitudinal mode can be increased. Furthermore, since the light 50 passes through the ring resonators 20a and 20b only once, the light intensity of the ring resonators 20a and 20b can be suppressed. As a result, multimode oscillation can be further suppressed, and the output power of the laser beam 60 can be further increased. In Example 1, the maximum output power of the laser beam 60 can be 40 mW or more by setting the light intensity of the ring resonators 20 a and 20 b to 40 mW or less.

  In order to shorten the resonator length, the number of the plurality of ring resonators 20a and 20b is preferably two. In order to make the path 56 symmetrical when viewed from the branch point 58, the AMZI 22 is preferably disposed between the pair of ring resonators 20a and 20b.

  By making the frequency interval FSR4 of the AMZI 22 about twice the frequency intervals FSR1 and FSR2 of the link resonators 20a and 20b, laser oscillation in the mode M2 can be further suppressed. In order to suppress the mode M2, the frequency interval FSR4 of the AMZI 22 is preferably 1.5 times or more and 3 times or less of the average value of the frequency intervals FSR1 and FSR2 of the plurality of ring resonators 20a and 20b, and 1.75 times or more and 2.5 times or less is more preferable.

  In the first embodiment, the AMZI 22 is described as an example of the transmissive portion. However, if the mode M2 is suppressed compared to the modes M1 and M3, the transmissive portion may be another element. For example, a ring resonator can be used as the transmission part. However, as shown in FIG. 5A, the AMZI 22 can increase the resonance width (that is, the passing width) as compared with the ring resonators 20a and 20b. For this reason, it becomes easy to superimpose the peak of transmittance AMZI of AMZI22 on the peak of R1 + R2 of mode M1.

  As shown in FIG. 9, the heater 34a (wavelength changing unit) changes at least one resonance wavelength of the plurality of ring resonators 20a and 20b. Thereby, the wavelength of the mode M1 can be changed.

  In the first embodiment, the example in which the heater 34a changes the temperature of at least one of the ring resonators 20a and 20b has been described as an example of the wavelength changing unit. The wavelength changing unit may be other means. By using the heater 34a as the wavelength changing unit, the wavelength of the mode M1 can be easily changed.

  The plurality of ring resonators 20 a and 20 b, AMZI 22, optical coupler 26, and optical waveguides 12, 14 a, 14 b, 16 a, and 16 b are optical waveguides formed on the same substrate 40. Thereby, the wavelength filter 10 can be reduced in size.

  Further, a core made of silicon and a clad made of silicon oxide are used. Thus, the difference in refractive index between the core and the clad is increased. Thereby, the light is easily bent in the ring resonators 20a and 20b. For this reason, the curvature radius r can be made small. Therefore, the wavelength filter 10 can be reduced in size. In order to oscillate in a single mode, the width W of the silicon thin wire 44 is preferably 500 nm or less and the height H is preferably 300 nm or less.

  In order to increase the transmittance difference between adjacent modes (M1 and M2) in the L band or the C band, the frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b are preferably 500 GHz or more. For this reason, the radius of curvature r of the ring resonators 20a and 20b is preferably 20 μm or less, and the circumference is preferably 200 μm or less.

  As the radius of curvature r decreases, the light does not bend and the loss increases. For this reason, the radius of curvature r is preferably 5 μm or more, and the circumference is preferably 100 μm or more. The frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b are preferably 1000 GHz or less. The coupling efficiency between the optical waveguide and the ring resonator is preferably 0.1 or more and 0.3 or less. A large coupling coefficient improves the maximum output power. On the other hand, when the coupling coefficient is small, the mode gain difference becomes large.

  In order to make the AMZI22 frequency interval FSR4 approximately twice the frequency intervals FSR1 and FSR2 of the ring resonators 20a and 20b, the path difference between the optical waveguides 24a and 24b in the AMZI22 is preferably 20 μm or more and 100 μm or less.

  As the optical coupler 26, a Y-branch type or multimode coupler can be used. The branching ratio of the optical coupler 6 is preferably 40% or more and 50% or less.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 Optical amplifier 12, 14a, 14b, 16a, 16b Optical waveguide 20a, 20b Ring resonator 22 AMZI
26 Optical coupler 34a-34c Heater 54, 55, 56 Path 58 Branch point

Claims (10)

The light output from one end of an optical amplifier having a gain band is arranged in cascade in the path where the light propagates so that the light is input to the one end, the frequency intervals are different from each other, and the total transmittance is within the gain band. A plurality of ring resonators having a highest first peak, a second peak adjacent to the first peak, and a third peak adjacent to the second peak;
A transmission unit that is arranged in cascade with the plurality of ring resonators in the path, and has a transmittance at a wavelength of the second peak lower than a transmittance at a wavelength of the first peak and the third peak, and transmits the light When,
A wavelength filter comprising:   2. The wavelength according to claim 1, wherein the transmission unit is an asymmetric Mach-Zehnder interferometer having a frequency interval that is 1.5 to 3 times an average value of frequency intervals of the plurality of ring resonators. filter. The path is branched at a branch point, and the branched path is a loop path;
3. The wavelength filter according to claim 1, wherein the plurality of ring resonators and the transmission unit are arranged in cascade in the loop path.   The wavelength change part which changes the wavelength of the said 1st peak by changing at least 1 resonance wavelength of these ring resonators is provided, The Claim 1 characterized by the above-mentioned. Wavelength filter.   The wavelength filter according to claim 4, wherein the wavelength changing unit changes the temperature of at least one of the plurality of ring resonators.   The wavelength filter according to any one of claims 1 to 5, wherein the path includes an optical waveguide formed on the same substrate.   The wavelength filter according to claim 6, wherein the optical waveguide formed on the same substrate has a core made of silicon and a clad made of silicon oxide. An optical coupler for branching a first optical waveguide coupled to an optical amplifier having a gain band into a pair of second optical waveguides;
A pair of ring resonators respectively coupled to the pair of second optical waveguides, having a first peak with a different frequency interval and a highest total transmittance in the gain band;
Both ends are coupled to one end of a pair of third waveguides coupled to the pair of ring resonators, respectively, and the transmittance peak overlaps the first peak, and the frequency interval of the pair of ring resonators An asymmetric Mach-Zehnder interferometer having a frequency interval that is not less than 1.5 times and not more than 3 times the average value;
A wavelength filter comprising:   The wavelength filter according to any one of claims 1 to 8, wherein an output power of laser light emitted from the optical amplifier is 40 mW or more, and an optical intensity in the ring resonator is 40 mW or less. .   A laser comprising: the optical amplifier; and the wavelength filter according to claim 1.

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