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The D2-135 Offset Phase Lock is a device designed for precisely controlling the frequency difference between two lasers, enabling them to operate at a specific, stable offset frequency. It achieves this by locking the phase of one laser (the slave laser) to another (the master laser), based on an optical beat note generated between them. This system is particularly useful in applications requiring highly stable and controllable laser frequency relationships, such as in atomic and molecular spectroscopy, quantum optics experiments, and precision metrology.
The core function of the D2-135 is to generate an error signal from the beat note of two lasers and use this signal to control the frequency of one of the lasers, thereby maintaining a constant frequency offset. The process begins with two lasers, a master and a slave, which are initially temperature stabilized and operating at their desired current. These lasers should have frequencies within a few GHz of each other. If the laser frequencies are unknown, a spectroscopy module can be used to tune one laser to a known transition and then the other laser can be brought close to the same transition.
An essential component of the D2-135 system is a heterodyne module, such as the D2-150 or the newer D2-250. This module is responsible for combining the light from the two lasers to produce an optical beat note. The alignment of the lasers into the heterodyne module is critical. The lasers are typically oriented 90° relative to each other, with their beams directed towards the center of the first beam splitter within the heterodyne module. The D2-150, for instance, has three adjustable beam splitters: two for aligning one laser and one for aligning the other. The master laser, which often has fewer adjustments, is aligned first, passing straight through a beam splitter to the fiber. The slave laser, which reflects off two beam splitters, is then aligned.
Once the lasers are aligned into the heterodyne module, a power meter is used to measure the power coupled into the fiber. For the first laser alignment, the goal is to maximize the power from the laser with one adjustment that gets coupled into the fiber, aiming for approximately 500 µW. It's crucial to avoid exceeding 1 mW input power to prevent damage to the detector in the D2-160 or D2-135-FC. If no light is detected, light can be coupled into the fiber in the opposite direction to help align the beam directly back to the laser source.
For the second laser alignment, once a few hundred microwatts of power from the first laser are coupled into the fiber, the second laser is unblocked. The last beam splitter in the heterodyne module has two outputs: one to the fiber and an exit port used for alignment. By observing the output at this port, both close to and far from the D2-150, the position and angle of the second laser can be matched to the first. This involves iterating adjustments of the beam splitters to achieve a good overlap and couple a few hundred microwatts of power from the second laser into the fiber.
After successful optical alignment, the next step is to find the beat note. The fiber output from the heterodyne module is connected to either the D2-160 or D2-135-FC. If using the D2-160, a short RF SMA cable connects it to the D2-135-SMA. The D2-135's Beat Note div/2 monitor is then observed on a spectrum analyzer. A beat note should appear at half the frequency difference between the two lasers. Laser current adjustments may be needed to bring the frequency difference into range. Once a beat note is visible, the laser frequency is adjusted until the beat note broadens or disappears. At this point, the laser is parked, and the alignment of the three beam splitters is tweaked until a clear beat note is observed. This process is repeated until the desired frequency offset, typically up to ~10 GHz, is achieved.
Finally, the D2-135 is locked. This involves connecting the D2-135's Servo Out to the Laser Controller's Servo In to sweep the slave laser's frequency. The Ramp TTL on the D2-135 is connected to the O-scope trigger, and the Error In Monitor is viewed on the O-scope. With the D2-135 set to N=16, VCO Low mode, and the Servo in Ramp mode, an error signal with a steep slope at the desired offset frequency should be visible. The reflection point on the error signal indicates where the two lasers are at the same frequency. The lock point is the steep slope to the left or right of the zero-offset point, which can be adjusted by changing the VCO frequency. The O-scope is centered on one of the lock points, ramp amplitude is reduced, and the D2-135 Servo is switched to Lock mode. If the Servo Output jumps to +/-10V, the gain sign is flipped. At this stage, a basic lock should be established, holding the offset frequency to a specific value, although the laser might be broadened due to servo oscillation.
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