Model No. D2-210
Document Revision: 1
Document Last Updated on 2019/01/23 18:39
The D2-210 saturated absorption spectroscopy module provides error signals derived from saturated absorption spectroscopy of atomic rubidium, cesium, or potassium. It contains a vapor cell, internal temperature controller, balanced photodetectors, and optics. Temperature control stabilizes the number density of atoms in the cell, and a balanced photodetection circuit compensates for intensity drifts giving stable control over the lock point for side locking applications. The photodiode output is shot-noise limited out to greater than 5 MHz for photocurrents of 50 μA and above. The high bandwidth of the feedback enables tight solid locking that is immune to vibrations and shock. The D2-210 is powered by a cable that plugs into the power connectors on any D2 series electronics module.
The D2-210 is a significant upgrade to the D2-110 incorporating many suggestions from our customers:
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Note: All modules designed to be operated in laboratory environment
The D2-210 requires +5 and ±15 VDC and ground to operate. The power input is via a female 6-pin Hirose connector HR10A-7TR-6SA(73). Depending on your order configuration, you should have received a power cable with this connector on one end and either the same on the other end (typically for use with ICE products) or a DB-9 connector (for use with D2-products).
If you are using the DB9-to-Hirose cable, but not a Vescent D2-005 power supply, table 3 below details which pins on the Hirose require which voltages.
Input light passes through an adjustable λ/2 waveplate and polarizing beamsplitter (PBS) that directs a user-controlled portion of the beam to the vapor cells and detectors. (For the fiber-coupled option the user will generally input most of the light.) The beam is then passed through a cleanup PBS and split on a 50:50 non-polarizing beamsplitter (NPBS). One portion is then passed through a reference vapor cell (Doppler subtraction option) or passed directly to the reference photodiode.
The remaining portion is passed through a fixed λ/2 waveplate and PBS to split off the pump beam from the probe beam. The pump beam is directed to the Output Optics assembly where it is combined with the probe beam on a PBS in a counter-propagating configuration. The pump beam is dumped through the reflection port of the PBS that separated the pump and probe and terminates on the wall of the housing. The probe is detected on the signal photodiode.
The beam diameters were designed to provide enough photocurrent (~50-100μA) to give shot-noise-limited performance out to 5 MHz while limiting saturation broadening. For Vescent's DBR lasers, the bandwidth is useful to provide for tight and stable locking by feedback to injection current.
Input Connector (6-pin circular)
Never connect this device to its power supply when the power supply is switched on and supplying power. Always turn off the power supply, make connections to the device, and then re-energize the power supply.
Power and temperature control signals from the D2-series power bus, the ICE-PB1, or other power supply are made through a 6-pin circular connector (HR10A-7TR-6SA(73)). The pin definitions (pin numbers are marked on the connectors) are listed below. figure 3 below may be useful.
Note: When powering the D2-210 with supplies other than a D2-005, pins 5 and 6 can be placed between 7-15 V. They need not be symmetric, but both supply rails are always required. See D2-005 manual for pin out on D-Sub 9-pin connector.
Signal Output (SMA)
The photodiode signal is output through an SMA connector. Connect to ERROR INPUT on the Laser Servo.
Sets the fraction of the Reference photocurrent to be subtracted from Signal. Setting the lockpoint close to zero Volts will give the best subtraction of relative intensity noise (RIN) on the incoming laser.
10-turn trim pot that sets temperature from 25°C (fully CCW) to 65°C (10 turns CW). The 5-turn point is approximately 42°C. Note that the vapor cells are heated only, so setting the temperature below room temperature essentially disables temperature control. The following table gives approximate temperature settings for the three available alkali options:
Temperature Loop Gain Adj.
In newer units, there is a middle trimpot for adjusting the gain on the PID temperature loops. Higher gain (clockwise) will result in faster temperature settling until oscillation occurs.
Temperature Bias Adj.
For the Doppler subtraction option this control enables the user to input a small temperature difference between the reference and signal vapor cells. In the fully CCW position the signal vapor cell heater gets all the current. In the full CW position the reference cell heater gets twice the current as the signal cell. This adjustment can be used to place the optimum point for Doppler Subtraction closer to zero Volts.
This adjustment only has an effect with the -DS, Doppler Subtraction option.
A polarizing beamsplitter (PBS) is fixed to a ball joint formed by a truncated steel ball held by a conformal brass ring. Two cone screws clamp the ring against the ball and fix the position of the PBS. See Alignment Procedure below for instructions on how to align and fix the PBS.
A waveplate attached to the front plate by magnets can be used to select the splitting fraction of the Tilt Plate PBS.
Note: When coupling the laser into the fiber, strive to properly align the laser polarization to the polarization axis of the PM fiber. Otherwise you will observe significant temperature and stress induced birefringence leading to input power fluctuations.
The spectroscopy module is shipped factory aligned and should not require alignment if ordered with a matching D2-100-DBR laser or if ordered with the fiber input (-FC) option. If the unit was not aligned to a laser at the factory, or if it falls out of alignment, the following steps can be used:
The following images are representative of spectroscopy for 87Rb with and without Doppler subtraction. Note that Doppler subtraction has a minimal effect on peak lock signal generated by dithering the laser (green traces).