CN105914566A - Device for utilizing birefringence of crystals to compensate time delays of ultra-short laser pulses - Google Patents

Abstract

The invention belongs to the technical field of ultrashort pulse laser, in particular to a device for compensating ultrashort laser pulse time delay by using crystal birefringence. The device for compensating ultra-short laser pulse time delay designed by the present invention is composed of two birefringent crystals whose optical axes are perpendicular to each other. By rotating the crystal to change the laser incident angle, the accumulated optical path of lasers of different frequencies can be changed, thereby changing the relative delay. The device of the invention is low in cost, small in volume, and does not need to split light; only by changing the incident angle by rotating the crystal, can realize continuous compensation for the relative time delay in the infrared band of 4 to 12.5 microns with femtosecond precision. Experiments show that after adding this device, the output efficiency of the commonly used femtosecond difference frequency system (one of the brands with the highest market share) can increase to more than 160%.

Description

A Device Using Crystal Birefringence to Compensate Ultrashort Laser Pulse Time Delay

technical field

The invention belongs to the technical field of ultrashort laser pulses, and in particular relates to a novel ultrashort laser pulse delay compensation device.

Background technique

Infrared spectroscopy is one of the most important experimental measurement techniques in scientific research and industry. Ultrafast time-resolved infrared measurements using ultrashort pulse lasers occupy a very important position in the frontiers of many disciplines such as physics, biology, and chemistry.

Optical difference frequency is one of the most important methods to generate ultrashort infrared pulses. It is a second-order nonlinear optical effect: when two beams of monochromatic intense light of different frequencies (referred to as signal light and idler light) are simultaneously incident on a nonlinear optical crystal, they pass through the second-order nonlinear optical effect of the crystal. A phenomenon in which linear optical polarizability couples to produce difference-frequency light whose frequency is the difference between two incident light frequencies. If there is a relative time delay between the signal light and the idler light, the two pulses cannot completely overlap in the time domain, and the efficiency of difference frequency generation will decrease. We found in experiments that even in a mature commercial laser system, there will be a relative time delay between the output signal light and the idler light, thus reducing the output power of the difference frequency light.

The traditional solution to this kind of problem is to use a beam splitter to separate the signal light and the idler light, let them pass through different optical paths, compensate for the relative time delay by changing the optical path difference, and then combine the beams to generate a difference frequency. This method has many limitations in practical application. First, beam splitters (beam combiners) that can efficiently separate signal light and idler light are required, including dichroic beam splitters or polarization beam splitters. Among them, the dichromatic beam splitter is difficult to distinguish signal light and idler light with very similar wavelengths, so it cannot be used to generate infrared pulses with longer wavelength bands; while the polarization beam splitter for ultrashort pulse lasers is often only suitable for specific wavelengths. Difficult to use in situations where continuous tuning is required. In addition, this type of device requires a micron-scale precision translation stage, as well as a variety of optical components and mechanical devices, which are expensive, take up a lot of space, and are often quite cumbersome to adjust.

In order to solve the time delay problem in the difference frequency generation and overcome the limitations of traditional methods, the present invention designs a new compensation device: it is composed of two birefringent crystals whose optical axes are perpendicular to each other, with low cost and small volume, and does not require Spectroscopy; only by changing the incident angle by rotating the crystal, continuous and femtosecond-level compensation can be realized for the infrared band from 2.5 to 16 microns and the relative time delay.

Contents of the invention

The object of the present invention is to provide a low-cost, small-volume device for compensating ultra-short laser pulse time delay without splitting.

The device for compensating ultrashort laser pulse time delay provided by the present invention utilizes the birefringence principle of crystals, and mainly includes: two birefringent crystals, the optical axes of which are perpendicular to each other. Among them, the optical axis of one crystal is parallel to the horizontal plane, and the other crystal is perpendicular to the horizontal plane as an example. Its structural principle is shown in Fig. 1.

In the present invention, in order to ensure sufficient time delay and avoid excessive broadening caused by dispersion, the thickness of the crystal is selected to be between 0.5-1.5 mm.

Signal light and idler light propagate in the same direction, enter the first crystal at an incident angle θ ₁ , and then enter the second crystal at an incident angle θ ₂ to exit. In the commonly used difference frequency generation system, the polarizations of the signal light and the idler light are orthogonal to each other, so different total optical paths are accumulated in the two birefringent crystals, resulting in an additional relative time delay.

Let the polarization direction of the signal light be perpendicular to the horizontal plane. In the first birefringent crystal, its polarization direction is perpendicular to the main plane formed by the optical axis and the direction of light propagation, so it is ordinary light, and the refractive index is only related to the wavelength of the signal light, and has nothing to do with the incident angle. In the second crystal, its polarization direction is parallel to the main plane, which is extraordinary light, and its refractive index is different from that of ordinary light. However, since the direction of the optical axis does not change with the incident angle, its refractive index is only related to the wavelength, and has nothing to do with the incident angle. .

The polarization direction of the idler light is parallel to the horizontal plane. In the first birefringent crystal, its polarization direction is parallel to the principal plane, which is extraordinary light. Since there is an included angle between the polarization direction and the optical axis direction, and the included angle changes with the incident angle, the refractive index is not only related to the wavelength, but also related to the incident angle. In the second birefringent crystal, the polarization direction is perpendicular to the main plane, which is ordinary light, and the refractive index is only related to the wavelength and has nothing to do with the incident angle.

In this way, when the two crystals are rotated (the rotation axis is perpendicular to the horizontal plane), the incident angles θ ₁ and θ ₂ are changed, and the relative time delay of signal light and idler light is changed at the same time. By selecting the thickness and material of the wafer, an appropriate combination of θ ₁ and θ ₂ can be found to fully compensate the original relative time delay.

In addition, in the present invention, two pieces of birefringent crystals are placed in the shape of "eight", so that the positional deviation generated after the beam passes through the wafer is canceled successively, ensuring the coincidence degree of the beam after passing through the compensator.

The device for compensating the ultrashort laser pulse time delay proposed by the present invention does not need a mechanical translation stage and a light splitting element.

In the device for compensating the time delay of ultrashort laser pulses proposed by the present invention, the frequency of two ultrashort laser pulses can be infinitely close.

The device for compensating ultrashort laser pulse time delay proposed by the invention can also be used as a precise time delay generating device in ultrafast dynamics measurement.

Beneficial effect

The device of the present invention has low cost, small footprint, convenient operation, high adjustment accuracy (femtosecond level), and is suitable for a wide range of wavelengths: there is no limit to the frequency difference between signal light and idler light; when used in After the Ti:Sapphire laser is used as the optical parametric amplifier of the pump source, it can compensate for the infrared difference frequency generation of 3-12.5 microns, and the efficiency can reach up to 165% of the original.

Description of drawings

Figure 1: Schematic of the device.

Figure 2: The agreement between the experimental data points and the theoretical delay contour map. Among them, (a) the difference frequency light wavenumber is 2000 cm ^-1 ; (b) the difference frequency light wavenumber is 1400 cm ^-1 .

Figure 3: Ratio of difference frequency light generation efficiency with and without compensator.

Figure 4: Shape comparison of ultrashort laser pulses with and without compensators.

detailed description

In order to verify and measure the compensation effect of this device, we conducted the following experiments.

As shown in Figure 1, a set of optical parametric amplifiers (the system is the market share of One of the highest brands), the frequency of the outgoing signal light and idler light is in the range of 1200-2600 nm, where the polarization direction of the signal light is perpendicular to the horizontal plane, and the polarization direction of the idler light is parallel to the horizontal plane. The two beams of light first enter the compensator, and then enter the difference frequency crystal to generate difference frequency light in the infrared band. After being filtered by a germanium chip, the difference frequency light enters the optical power meter (Thorlabs S302C). The efficiency of difference frequency generation is affected by the relative time delay between signal light and idler light. Therefore, the compensation effect can be measured by measuring the change of difference frequency optical power before and after adding the compensator.

1. Theoretical calculation

The wavelengths of pump light (pump), signal light (signal), idler light (idler) and difference frequency light (DFG) have the following relationship:

.

in, is the wavelength of the laser pump light, which is 800 nm in the experiment, and are the wavelengths of signal light and idler light generated from the OPA, It is the wavelength of the difference frequency light generated by the optical difference frequency between the signal light and the idler light.

According to the knowledge of birefringence, the crystal The refractive index of light is independent of the direction of propagation and is always ; The refractive index of e light is related to the propagation direction, and the refractive index along the optical axis is , the refractive index in other directions and the propagation direction are related to the angle (θ) of the optical axis, and the specific relationship is as follows:

Combined with the law of refraction of light:

Available:

.

Among them, θ _i is the incident angle, _ni is the refractive index of the medium before entering the crystal, n _to and n _te are the refractive indices of ordinary light and extraordinary light in the crystal, respectively.

A beam of light enters a piece of crystal with a thickness of d at an incident angle _θi and then exits; compared to directly passing through a vacuum of the same thickness at an incident angle _θi , the relative time delay generated can be expressed by the following formula:

.

Among them, _nt is the crystal refractive index, d is the crystal thickness, _θi and _θr are the angle of incidence and refraction, respectively, and c is the propagation speed of light in vacuum. Using the above formula, the signal light and the time delay generated by the signal light after passing through the compensator relative to the vacuum of the same thickness can be calculated, and the subtraction of the two is the time delay generated between the signal light and the idler light by the compensator additional delay.

It is calculated that, for the wavelength band used in the experiment, when passing through a piece of birefringent crystal with a thickness of 1 mm, the maximum time delay that can be generated is 30 fs; when the incident angle is around 45°, the incident angle changes 1°, the delay changes by about 5fs.

Figure 2 is a graph of the calculated extra time delay generated by the compensator. (a) and (b) are the values when the infrared wavenumbers are 2000 cm ^-1 and 1400 cm ^-1 respectively. The abscissa in the figure is the incident angle θ ₁ of the two beams at the first crystal, and the ordinate is the incident angle θ ₂ at the second birefringent crystal. _Calculate the additional time delay ₍ ). The curves in the figure are a series of "contours", corresponding to the value range of (θ ₁ , θ ₂ ) that produces a specific additional time delay. Here, when the delay of the signal light is greater than that of the idler light, take sign is positive. According to the parameters we choose, It can be continuously changed from negative value to positive value through zero. When there is a certain time delay between the signal light and the idler light incident on the compensator, a combination of a series of incident angles can be found to obtain an additional time delay opposite to the original time delay, so that the two beams of pulses are strictly separated when they are emitted. Synchronization, thereby improving the efficiency of difference frequency generation.

2. Experimental results

(1) The compensator improves the difference frequency power

In the experiment, for each wavelength of difference frequency light, we first measure the difference frequency optical power before adding the compensator, then add the compensator to the optical path, fix θ ₁ , adjust θ ₂ , and record when the difference frequency optical power reaches the maximum value Three quantities: θ ₁ , θ ₂ , and the difference frequency optical power at this time; then, we change the value of θ ₁ , adjust θ ₂ again until the difference frequency optical power reaches the maximum again, record again, and repeat the above steps.

The experimental results are as follows. First of all, in different infrared difference frequency bands and using different difference frequency crystals, the compensator of the present invention can play a compensating role, and the output power can reach more than 160% of that without the compensator (as shown in Table 1 and Figure 3 ).

surface Different Band Compensation Effects

.

(2) Pulse shape comparison

Due to the dispersion of the crystal, after passing through the compensator, the pulses of the signal light and the idler light will be broadened, and the spectral width of the difference frequency light may be narrowed. In order to observe the spectrum of the difference frequency light before and after adding the compensator, we overlap it with another narrow pulse (4.2ps) with a wavelength of 800 nm on the nonlinear crystal, and measure and observe the spectrum of the sum frequency light generated by them. Since the spectral lines of 800 nm pulses are much narrower than those of infrared pulses, their sum-frequency spectra reflect the spectra of infrared pulses themselves. As shown in Figure 4, the abscissa is the wave number of infrared light, and the ordinate is the intensity of the sum-frequency light. Before and after adding the compensator, the shape and width of the sum-frequency spectrum are almost unchanged, which proves that our compensator will not reduce the difference frequency light. spectral quality.

(3) Verification of compensation principle

In order to verify that the effect of power enhancement really comes from the compensation of the relative time delay between the signal light and the idler light, we plot the (θ ₁ ,θ ₂ ) combination when the difference frequency light power is maximized as data points in Fig. 2 . It can be seen that all data points are very close to the calculated "equal delay line", which confirms that there is indeed a relative time delay between the signal light and the idler light output by the laser system, and the device of the present invention can compensate for this time delay , and effectively improve the difference frequency generation efficiency.

Claims (4)

1. A device that utilizes the birefringence of crystals to compensate ultrashort laser pulse time delay, is characterized in that it mainly includes: two birefringent crystals, the optical axes of the two crystals are perpendicular to each other, and the optical axis of one of the crystals is parallel to the horizontal plane , and the other piece is perpendicular to the horizontal plane. 2. The device according to claim 1, characterized in that the thickness of the two birefringent crystals is 0.5-1.5 mm. 3. The device according to claim 1, characterized in that: by rotating the crystal to change the incident angle of the laser, the optical path in which lasers of different frequencies are accumulated is changed, thereby changing the relative time delay. 4. The device according to claim 1, characterized in that: the two pieces of birefringent crystals are placed in a "eight" shape, so that the positional offsets generated after the beam passes through the wafer are offset successively to ensure that the beam passes through the compensator degree of coincidence.