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Lab is used for long-term frequency stabilization of lasers

Lab is used for long-term frequency stabilization of lasers


▍Main features
Signal processing block diagram

The signal is demodulated using internal and external native oscillators

Sawtooth or triangular resonance scanning

Use the built-in oscilloscope to observe signals at different locations during signal processing

Use the "click-to-lock" function to quickly lock to any zero crossing point of the error signal

Up to fourth-order low-pass IIR infinite impulse response filter demodulation signals

Individually configurable high-bandwidth and low-bandwidth PID controllers are used for high-frequency and low-frequency feedback

Use the Scan Lock in Range function to observe signals related to the scan voltage

▍Typical parameters
The frequency of the local oscillator is 1 mHz -200 MHz, and the frequency resolution is 3.55 μHz
Scanning waveforms: positive sawtooth, negative sawtooth and triangle wave
Sweep frequency: 1 mHz -1 MHz
Low-pass filter cut-off frequency: 1 kHz - 14 MHz
Filter types available: Butterworth, Chebyshev, Inverse Chebyshev, Elliptic, Gaussian, Bessel, Legendre
AC/DC input coupling
50 Ω / 1 MΩ input impedance

The following application note describes how to achieve long-term laser frequency locking with the Moku laser frequency-locking/stabilizing instrument.

Laser locking systems are widely used to control and match the frequency of the laser to an optical frequency reference (this is usually an optical reference cavity or atomic transition). Such systems are essential for high-resolution interferometry, spectroscopy systems, and time and frequency standards.
There are generally two cases in which a laser is locked by forcing the laser and the reference frequency to be equal: (1) the locking system controls the laser frequency and makes it equal to the reference frequency, which is known as frequency stabilization; (2) The locking system forces the reference frequency to follow the laser frequency, which is known as frequency tracking. Whether for frequency stabilization or frequency tracking, Liquid Instruments' Moku enables a high-performance, high-gain laser locking system. Moku offers advanced setup, acquisition, and diagnostics that make setting up and characterizing a laser lock system easier and faster.


▍Basic knowledge of laser locking and PDH technology
At the heart of any laser locking technique is to measure and provide a measurement of the difference or error between the laser and the frequency reference. Often referred to as an "error signal", the quality of this signal ultimately determines the precision and accuracy of the entire locking system. Arguably, one of the most accurate ways to obtain an error signal is the Pound-drever-Hall (PDH) technique. It has been proven that the use of PDH error signals in feedback systems can be used to measure changes in lasers or cavities with great precision, allowing them to be used in a myriad of applications such as absorption spectroscopy and gravitational wave detection. There are several key advantages of PDH error signaling technology, such as:

1. This technology can accurately measure and provide the phase and frequency difference between the laser and the resonant cavity

2. This sensing technology provides a zero-cross-error signal, and when the error signal is zero, it means that its zero frequency difference is zero.

3. Assuming that all signal processing is done digitally, it avoids the low-frequency noise generated in analog electronics and demodulation circuits.

These advantages inevitably come at a price. To obtain this precise measurement of frequency/phase, PDH technology applies RF modulation and demodulation techniques. This greatly increases the complexity of the signal processing system and also complicates the optical system. However, once understood, these complexities are insignificant compared to the advantages of the PDH system.

▍ Use Moku: laser frequency-locking/frequency-stabilizing instrument to achieve laser locking
Moku: Laser locking/stabilizing greatly simplifies the complexity of typically operating and using a PDH locking system. Figure 1 shows an example of a PDH laser locking system. The device uses a solid-state Nd:YAG NPRO laser, which has been collimated and pattern-matched to a moderately fine optical cavity. This is followed by the use of Moku: Laser Frequency Locking/Frequency Stabilization to generate all the signals needed to lock the laser to the resonant frequency of the cavity.

                        Figure 1   An example of a PDH laser locking system

Locking lasers include:

1. Set up the system (including collimation).

2. Modulated laser

3. Look for resonance points

4. Obtain an error signal

5. Open Feedback

6. Optimize locking

▍ System settings
In order for the system to work optimally, it is necessary to ensure that the outgoing light of the laser is well aligned with the optical axis of the cavity and that the pattern of the laser matches well with the spatial pattern of the resonator. It is important to note that malalignment or pattern mismatch can lead to reduced locking performance or, in extreme cases, the system simply not working. Finally, two photodetectors are used to monitor the system; One photodetector receives the light reflected back from the cavity, and the other receives the light that passes through the cavity.

▍Connect to the Moku:Lab output
In order to successfully lock the cavity with PDH, several signals need to be generated.

1. Modulated Signal: Sent to EOM to generate a phase-modulated sideband.

2. Primary feedback signal: In this case, the PZT frequency controller that feeds back to the laser. In order to drive the PZT of the laser, a high-voltage amplifier (HV amp) is required.

3. Secondary feedback signal (optional): The laser frequency can be adjusted by temperature, and the dynamic control range of temperature feedback is wide, but the speed is slow.

In this case, the modulation signal and the secondary feedback signal are generated on output 2 of Moku:Lab and separated using Bias-Tee.

▍Connect to the Moku:Lab input
The reflected signal received by the photodetector usually contains all the information needed to generate a feedback signal, which is connected to input 1 and used as the primary signal input channel. The second input channel can be used to monitor any auxiliary signal.

1. Input 1 is used as the primary channel for most signal processing. In this system, the photodetector AC output is connected to input 1 of the Moku:Laser Lock Box.

2. Connecting the DC component of the transmitted light signal to input 2 helps to identify and optimize features in the locking system, although not required.

▍ Modulated laser
Phase modulation in this case is achieved by applying a sinusoidal voltage signal to the EOM. The modulated signal can be generated by the auxiliary oscillator function. For this system, we will use 10 MHz modulation.

1. Set the auxiliary oscillator to 10 MHz.

2. Set the amplitude of the auxiliary oscillator. Be sure to select a voltage that is within the EOM specification range. In this case, we set the amplitude to 100 mV.

3. Select the Aux Oscillator output. In this example, the Aux oscilloscope is set to output 2.

4. Turn on the output.

▍Scan the laser frequency and find the resonance frequency
Scanning laser frequencies help characterize and optimize locked signals.

Moku: The Laser Lock Box comes with a scanning function. In this example, we will set the scan generator to output a signal and pass it to the PZT actuator via output 1. The steps are as follows:
1. Set Scan to a triangular wave

2. Set the amplitude to 500 mV

3. Select the output of the scan signal, in this case Output 1

4. Turn on the output

                Figure 2   An auxiliary oscillator is used to drive the EOM and create a

                                                    phase-modulated sideband.

▍ Center the system resonance signal
In order to make the laser lock smoother, we can usually adjust the resonant signal to the center of the scan signal during the scan, and later by adjusting the offset of the temperature controller.

o Adjust the temperature offset until the scan center resonance frequency appears near the zero value of the scan signal.

▍Acquire and optimize error signals
In order to obtain an error signal, the RF signal received from the photodetector needs to be demodulated with a local oscillator. Choosing the correct phase of the local oscillator is critical to optimizing the error signal. As follows:

o Adjust the phase of the local oscillator while observing the error signal.

Figure 3      Channels A and B show the transmitted response of the cavity and the

                                            recovered error signal, respectively.

▍ Manually lock the laser
1. Adjust the resonance signal to the center of the scan.

2. Set up the high-frequency PID controller. (Here you can only set the integrator frequency to ~10 Hz first, as the response can be further optimized later)

3. Turn on the PID controller

4. Slowly reduce the scanning amplitude until the laser power reaches its maximum value.

5. Turn off scanning

▍ Use Tap to lock
1. Adjust the resonant signal to the center of the scan.

2. Set up the high-frequency PID controller. (Here you can only set the integrator frequency to ~10 Hz first, as the response can be further optimized later)

3. Select Tap as the lock mode

4. Select the resonance point you want to lock

Note: Make sure the feedback is in the right direction.

▍ Adjust and optimize locking
Once the system is locked, we can optimize the locking as needed. This usually means adjusting the gain of the PID controller.
To do this, open the PID controller menu:
1. Increase the proportional gain slightly until the system starts oscillating.

2. Reduce the proportional gain slightly until the system stops oscillating

3. Repeat for integrators and differentiators (if necessary)

Figure 4     When the laser is locked, the power of the transmitted light (channel A)

    will be at its constant maximum. The error signal (channel B) will also remain zero.

In addition to laser frequency locking/frequency stabilization, Moku:Lab also integrates 12 professional instruments: oscilloscope, spectrum analyzer, waveform generator, phase meter, data logger, lock-in amplifier, PID controller, Bode analyzer, digital filter, arbitrary waveform generator, and FIR filter generator.

 

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