Laser Diode Driver, Ultra Low Noise, 500 mA, Battery Operated

Each laser diode driver’s laser current source can be operated in one of three modes:

• Constant current, low bandwidth: optimized for DC operation and offers improved laser protection and lowest noise.
• Constant current, high bandwidth: the output stage supports up to 1MHz modulation frequency for dithering the laser current in power and wavelength tuning applications.
• Constant optical power: provides constant optical power operation of your laser diode by measuring the photocurrent from the laser diode’s rear-facet photodiode or an external photodiode in a feedback control loop to the current source.

The current output of the LDX-3620B can be controlled by an external voltage source; either a DC signal or an alternating waveform for applications requiring DC biasing and/or linewidth broadening, wavelength control or amplitude modulation. One or two external voltage wave forms can be used to modulate the output current through the front panel modulation input ports. Both inputs can be AC or DC coupled to the current source drive circuits. The transconductance of each input differs by a factor of 10 for modulated and error signal summing to the drive circuits. In all modes of operation, the voltage inputs are summed with the front panel current controls for current control to the laser.

A lead acid battery and careful design and attention to detail in component selection, instrument and circuit board design allow the LDX-3620B to achieve an order of magnitude lower noise levels than AC powered current sources. With proper cabling and laser diode mounting, current noise levels of <100 nA can be achieved; critical performance for spectroscopic, coherent communications, and other applications requiring narrow linewidths. The same attention to detail also allows the LDX-3620B to achieve current source stability on the order of 10ppm over any one hour period in any instrument mode of operation.

Laser diodes are extremely sensitive to electro-static discharge, excessive current levels, current spikes, or transients from power surges or other laboratory equipment. One of the most important features we have implemented into each instrument is the ILX Lightwave proven laser diode protection standards. These standards have led to advanced protection features such as adjustable current limit and circuit design. During power up, the laser is protected from over current and turn-on transient conditions by a slow start routine designed to ramp the current to the desired set point referred to as slow-start. Current source response is tested to IEC surge and electrically fast transients (EFT) standards. The current source output is floating relative to chassis ground to prevent any potentially damaging common ground coupling between instruments or machinery and the laser diode. For more information about these protection features, please see our “Protecting Your Laser Diode" application note.

The LDX-3620B can be operated up to 16 hours on a full charge making it ideal for overnight testing. Monitoring the battery charge level any time during instrument operation is easy with a front panel battery charge indicator. Pushing the switch on the left of the display indicates a charge level up to 100% in 25% intervals. The internal batteries provide the benefits of lower noise without the drawbacks of battery powered instruments. Lead acid batteries with good charge retention and charge recoverability were chosen. A DC input to the instrument with a battery charge mode is provided for charging the batteries even during instrument operation. Low battery circuits were designed into the instrument to safely shut down the instrument if the battery charge gets below required instrument operating levels.