MBR 110

From Qwiki

MBR-110 (Monolithic Block Resonator (machined from a single piece of aluminum)) The first laser of its series to have been sold in the United States. It is a Titanium-Sapphire laser with some issues, but nothing that a little tender-loving can't solve. Both the MBR 110 and the pump Verdi are sold by Coherent Inc.

80kHz Etalon Noise

Although a birefringent filter and brewster plates allows you to coarse tune the Ti:Sapph wavelength, the etalon is necessary for fine-tuning and single-mode operation. The transmission of the etalon is locked to the laser frequency and this is what prevents mode-hopping while the laser is being fine-tuned. This lock is realized by piezo modulating the etalon at 81 kHz (probably a resonant frequency of our particular etalon) and directing the small reflected signal from the etalon onto a photodiode. This photodiode's signal can now be demodulate at the drive frequency to extract an error signal and thereby stabilize the etalon for maximum transmission (and presumably mode-hop free operation.)

Sounds easy, but unfortunately the fine people at Coherent seem to be incapable of designing a decent etalon error lock. They compensate for this by drastically over modulating the etalon to provide a more-than-adequately sized error signal. Consequently, the laser output frequency and amplitude nominally contain significant amounts of 81 kHz and 162 kHz noise, respectively. Lab measurements have found that the frequency modulation depth in this state can be as bad as 1 MHz (at the 81 kHz modulation frequency) and a relative amplitude noise as large as 5% (at the annoying 162 kHz frequency.)

After following information regarding the problem was obtained from communication with Coherent Inc and some experimenting:

Option 1:

Potentiometer PR-16 (located inside the laser electronics box) controls the size of the modulation signal to the etalon. Rotating this pot CCW will increase the modulation, thereby improving the quality of the etalon error signal at the expense of introducing additional laser frequency and amplitude noise. The modulation amplitude is nominally set large enough that the laser can be locked to the external cavity over a 20 GHz scan range. However, for more sensitive single-frequency operation the modulation drive can be considerably reduced whilst maintaining mode-hop free operation (i.e. the etalon will still lock). A few CW rotaions of PR-16 will do the trick: in practice the etalon will still lock as long as the etalon error "saw-tooth function" is about 100mV. However, the laser may no longer scan in this state, but that's life.

Option 2:

Tweeking the PR-6 potentiometer counter clockwise adjusts the gain of the "noise-eater circuit." This is supposed to help with 160 kHz second-harmonic noise. Unfortunately, all experience to date contradicts this advice. PR-6 appears to do nothing to the amplitude or frequency noise.

Additional information

The following is a list of components and test points within the laser electronics head:

TP-5: 81 kHz modulation drive signal

TP-11: The raw demodulated etalon error signal (the "etalon error signal" is essentially this signal, but stripped of the demodulation image frequency)

PR-6: Allegedly controls gain of a magical amplitude noise eater circuit

PR-10: Error signal demodulation (fine) phase adjust (90 degree range)

PR-16: Controls size of 81 kHz modulation drive to etalon

SW-10: Error signal demodulation (coarse) phase adjust (each dip switch setting represent a different phase quadrant)

IC-15: A dual op-amp (Phillips Semiconductor NE5532) that actuates the cavity tweeter mirror. Adding a signal (via a 20k resistor ) into the inverting pin (pin 2 on IC-15, where R37 and R39 meet) will allow feedforwards into the tweeter. The tweeter itself (or related circuitry) has a single resonance near 20 kHz, but is otherwise clean well up to ~100 kHz.) As a reference, 100mV onto the tweeter changes the laser frequency by ~ 100 kHz for modulation frequencies in the range 1 kHz - 100 kHz (the response is larger near the resonance, though.)