[Instrument usage skills] Working principle and adjustment method of autocorrelator (2)

[Instrument usage skills] The working principle and adjustment method of the autocorrelator (2)

The main content of this article is based on the autocorrelator of Coherent Inc. for 800nm wavelength (right picture), and explains its working principle and adjustment method. Figure 1 below is the optical path diagram of the autocorrelator:

Figure 1. (a) Optical path diagram and (b) actual structure diagram of the autocorrelator SSA of Coherent Inc.

1. Preparation work:

1. Since the energy of the light to be measured is relatively strong, a white plate is needed to weaken the energy of the light to be measured. Here, a wedge plate with a large wedge angle is used, and the reflected light from the front surface is used as the light to be measured (the light reflected from the back surface will cause other dispersion due to passing through the wedge plate material twice, causing the pulse shape and width to change)

2. Adjust the light to be measured to be horizontal to the table, and the light height is the center height of the wave plate.

Use a level to level the bottom surface of the autocorrelator SSA (if you do not move the SSA frequently, this step can be omitted)

2. Formal measurement:

1. Introduce the light to be measured into the SSA through the reflector M1, adjust the angle of the SSA on the table so that the direction of the light to be measured is perpendicular to the surface direction of the SSA, and fix the SSA with a pressing block;

2. Adjust the reflector M1 so that the light to be measured passes through the center of the wave plate λ/2, the beam splitter BS1, and the delayer DL lens in sequence. At the same time, the light to be measured will pass through the center of M3; the two beams of light split by BS1 are temporarily named transmitted light a and reflected light b;

3. Adjust the angle of the reflector M2 so that the reflected light b reflected by BS1 is incident on the center of the reflector M4; at this time, the reflected light b will pass through the beam splitter BS2. If the reflected light b does not pass through BS2, move the position of BS2;

4. Adjust the angles of the reflectors M3 and M4 so that both the transmitted light a and the reflected light b fall on the reflector M5, and are distributed up and down, consistent on the left and right; as shown in Figure 2. (a);

Figure 2. (a) Two light spots on the reflector M5; (b) Two light spots on the crystal; (c) Sum-frequency signal behind the crystal

5. Adjust the angle of the reflector M5 so that the transmitted light a and the reflected light b are incident on the non-linear crystal BBO in a non-collinear form; the two light spots will partially overlap, as shown in Figure 2. (b);

6. Verify, block the reflected light b, adjust the position of the delayer DL, and observe whether the position and direction of the transmitted light a on the crystal changes; if it changes, the delayer DL needs to be re-adjusted. If it does not change, continue to the next step;

7. Place a piece of paper behind the BBO and adjust the sum-frequency signal of the transmitted light a and the reflected light b, as shown in Figure 2. (c), which is a sharp blue line; the specific adjustment method is as follows:

a) Rotate the BBO crystal angle to α, so that the transmitted light a appears to self-double, that is, the transmitted light a becomes blue, and the reflected light b is still red light;

b) Rotate the BBO crystal angle to β, so that the reflected light b appears to self-double, that is, the reflected light b turns blue, and the transmitted light a remains red light;

c) Rotate the BBO crystal angle to (α+β)/2. At this time, neither the transmitted light a nor the reflected light b is self-multiplied; at the same time, the sum-frequency signal cannot be observed; at this time, move the position of the delayer DL until the sum-frequency signal appears after the crystal BBO, as shown in Figure 2. (c); If this phenomenon cannot occur, it may be that the optical path difference between the two lights is too great, and the optical path needs to be redesigned (this situation generally occurs with self-built measuring instruments);

8. After the sum-frequency signal appears, if the signal is too weak, rotate the half-wave plate by λ/2 to make the sum-frequency signal stronger;

9. Connect the sum-frequency signal to the oscilloscope through the output port of the SSA. If the signal light is too strong - the signal is saturated on the oscilloscope, you can add an attenuator behind the BBO crystal. Note that you cannot add an attenuator in the measurement light path and in front of the crystal, because the transmission element will cause the two paths of light to be unequal or change the chirp or pulse width of the light to be measured;

10. Read the width of the electrical signal pulse (FWHM) on the oscilloscope as shown in Figure 3.(a) The medium pulse width is 152.4 μs;

Figure 3. (a) The oscilloscope displays the electrical pulse (b) The movement of the electrical signal in time during calibration

11. Calibration:

a) Connect the trigger line;

b) Adjust the position L1 of the delayer so that the sum frequency signal moves to the leftmost end in time, as shown in Figure

3. At the position of the cursor a in (b), write down the delayer position L1 at this time, and use the oscilloscope light to mark the pulse center position a on the oscilloscope;

c) Adjust the delayer position L2 to move the sum frequency signal to the rightmost end in time, as shown in Figure 3. (b) At the position of the cursor b, note down the delayer position L2 at this time, and use the oscilloscope light to mark the pulse center position b on the oscilloscope;

d) Calculate the optical path movement position of the transmitted light a, and calculate the corresponding time movement of the electrical pulse in the oscilloscope

e) Fixed proportional coefficient, if the input pulse is a Gaussian pulse, then k=0.707

11. Calculate the actual size of the light pulse

12. If the pulse needs to be compressed, move the spacing of the compressor before the SSA until the measured optical pulse width is the smallest (shown as the sum-frequency blue line signal after the BBO is the thinnest and sharpest, and the pulse width displayed on the oscilloscope is the smallest).

3. The author reminds:

The calibration results have a great influence on the pulse measurement.

1. For different shapes of pulses, the scaling coefficients are different. If the laser pulse waveform is Gaussian, hyperbolic secant, or unilateral exponential, the conversion coefficients are 0.707, 0.648, and 0.5 respectively;

2. If the electrical signal pulse displayed by the oscilloscope has a base, it means that the pulse to be measured has high-order dispersion;

3. When calibrating, make sure that when moving the delay device DL, the electrical pulse displayed on the oscilloscope moves in time by more than 2ms;

4. When calibrating, make sure that the peak level of the electrical pulse displayed on the oscilloscope is above 500mV and the level is not saturated;

5. When measuring, all the light to be measured should enter the autocorrelator. If the spot to be measured is too large, the spot can be narrowed;

Autocorrelators for other wavelengths and structures, which have similar structures and consistent principles, can be quickly adjusted using this method.