Coherent 899-21 linewidth improvement

This page has been started as part of the process to document and present the modifications we have made to various components within the Coherent 899-21 laser system installed in our lab. Before implementing these changes we were measuring laser linewidths between 3 and 5 MHz (FWHM) using standard scanning and fixed-length cavity techniques with various reference cavities including the laser's own on board reference cavity. Recent measurements, after the following modifications, have been consistently within 0.8-1.0 MHz. Changes in the pump laser (see below) resulted in a linewidth of 0.25 MHz or 250 kHz.

Laser Configuration

Our Coherent 899-21 is configured with the Titanium Sapphire crystal and optics appropriate for wavelengths near 770nm. All measurements have been verified near both our references: the D1 and D2 Sodium lines (769.9nm and 766.7nm respectively). Initially a Coherent 300 Argon-Ion laser pumped the Ti:S but we have now switched to a Coherent Verdi V-8 for the pump laser. As expected, the new pump improved the laser linewidth.

Linewidth Measurements

Our measurements use the following general procedure:

Mode match the laser output to a cavity with known parameters (focal length, configuration, and/or FSR etc.)
Scan either laser frequency or cavity length and calibrate an oscilloscope scale using known or measured cavity FSR.
Measure the slope of a fundamental line present in the scan. This should correspond to a single longitudinal mode (or longitudinal and degenerate transverse modes in the case of a confocal cavity such as the 899-21's on-board reference cavity).
Without scanning either the laser or the cavity, adjust laser frequency or cavity length to obtain a signal that corresponds to "parking" the laser on the side of the line chosen in 3). The variation of the signal amplitude at this point can be translated into variation in frequency by using the slope and calibrated frequency information from previous steps.
The most specific method to obtain a FWHM linewidth measurement is to histogram the above data and make an appropriate data-fit.

Note: this process of using the known slope of a resonance to convert frequency variation into amplitude variation is analogous to one of the early FM radio detection schemes (see pg. 99 of John L. Hood's Audio Electronics for example).

Equipment Used

Coherent 240 scanning confocal cavity

Burleigh HiFase variable-length scanning cavity

Various Tektronix digitizing and analog oscilloscopes

Acoustic Considerations

For this and other research, it is useful to limit the amount of acoustic noise transmitted by instrument cases. We have seen marked improvement in many situations by simply covering instruments and controllers with lead-core foam sheeting obtained from E. N. Murray Co.

Laser & On-board cavity

We applied lead-core foam to the removable laser cover in order to limit transmission of acoustical noise from the room into the cavity space. Additionally we wrapped smaller pieces of foam around the reference cavity. We specifically covered the space between the cavity and the mounting hardware because these points of contact seemed inadequately isolated from the environment.

Measurement Cavities

Acoustic noise must be reduced on two fronts; we've limited noise at the laser but equally important is limiting acoustic noise introduced to the measurement cavity. The Burleigh HiFase cavity system is packaged as a closed system with a removable cover. As with the laser head, lead foam was applied to the outside of the removable cover. If space is adequate in either case it would be advantagous to apply foam to the inner surface in order to reduce internal acoustic resonance within the device case. Aditionally vibration reducing rubber (from Sorbothane) was used between the table surface and the HiFase case feet.

The Coherent 240 scanning cavity is a stand-alone reference cavity which allows for independent movement of the cavity relative to the laser source and associated optics. To reduce such relative movement the cavity, optics, and fiber mount were magnetically attached to 1/2" steel plating which in turn was isolated from the table with the vibration reducing rubber. Additionally we noted improvement after re-routing the optical fiber to avoid instrument fans and other sources of air currents or vibration.

Electronic Considerations

The 899-21 uses feedback control techniques to stabilize laser frequency based on the transmission through the on-board reference cavity. Similar to a basic speaker system that separates high frequency signals from low frequency signals, the corrections to the laser ring cavity are applied by two devices, one for each frequency range. The tweeter (high freq.) is a piezo-electrically controlled mirror and the woofer (low freq.) is a piezoelectric brewster plate. One section of the control circuitry is responsible for looking at the error signal and generating a control signal for each frequency device, woofer and tweeter. This specific section of the circuitry is located on the IA9 circuit board.

Navigating the 899-21 control box

The 899-21 control box (left) uses a modular circuit board design where 9 individual boards are plugged into a motherboard. The slots on the motherboard are labeled with each board identifier and name. Our modifications were all on the IA9 board which, if you are facing the control panel, is the right-most board in the box (pictured at right).

The IA9 board

This board is responsible for processing an error signal and sending the appropriate control signals to the high and low frequency correction devices (tweeter and woofer). The full schematic of this board is identical to that for the Coherent 699-21 a scan of which is included to the right.

Our only electronic modification was in the initial tweeter gain stage which uses amplifier A7. A closeup of the marked changes is shown below.

Circuit Changes

We changed the resistance of two resitors in the tweeter gain stage at amplifier A7. Resistors R16 and R44 were bridged with 36.5k resistors to create an effective resistance of half their original value (about 18k). Note this nearly restores the value specified in the Coherent 599 control circuit (15.0k) which is known to operate well at a narrower linewidth. These basic circuit modifications led to a dramatic increase in laser stability by increasing the stabilization circuit's response to errors in the laser's output frequency.

Andrew Dawes