EM657 Series

This module integrates a high-bandwidth fiber-coupled DFB laser with both an ultra-low noise laser current source and temperature controller, as well as an optical isolator and back facet monitor detector readout amplifier.

The entire module operates from a single +5 V supply and provides a bi‐directional power adjust input that may be used for SBS prevention, for constant power operation in conjunction with the monitor detector readout signal and an external control loop, or for finely adjusting the laser oscillation frequency via chirp. The unit also incorporates a bi‐directional temperature adjust input for coarse tuning of the laser oscillation frequency. The module is designed and built using G&H’s high‐reliability platform for defense components and incorporates an advanced ultra‐low noise laser current source.

¹ Power stability of this magnitude is strongly influenced by any movement of the fiber. To duplicate this stability measurement the fiber must be secured and motionless.
² Units are available without isolator. Devices without optical isolators are subject to mode-hops and are susceptible to back reflections. The wavelength stability devices with no optical isolator cannot be guaranteed.
³ The peak of the RIN curve corresponds to the relaxation oscillation frequency of the laser which varies in proportion to the drive current above threshold by f relax α ((I ld /I threshold ) - 1) 1/2. Customers employing this device in RIN sensitive applications should therefore be aware that reducing the bias level using the power adjust input will reduce performance. Reducing the bias level reduces the device modulation bandwidth by the same relationship.
⁴ Optional 900 μm loose-tube PVDF buffer recommended for laboratory use.
⁵ Other connector options available, contact sales for more information.

Pinout

Max Ratings

* Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only and operation of the device at or beyond these conditions is not implied. Exposure to absolute maximum ratings for extended periods of time may affect device reliability.

Application Notes

Mounting

The EM657 is conductively cooled through its base and needs to be mounted using a thermal interface material to a customer supplied heatsink. Do not use graphite sheets with adhesive backing as the adhesive is an insulator. Care should be taken to keep the base temperature of the module between -10 - 60°C at all times during operation.

Bias Level Adjust (PA)

The EM657 is designed to run in constant current mode with the drive current set for the as-ordered output power to achieve the highest possible performance. However, some applications require fine tuning of the laser bias current. The PA input provides this functionality, but its use carries an amount of risk. If bias adjustment is not required this input should be left open. Use of this input carries the potential to overdrive the laser with the ability to destroy or drastically reduce the device lifetime. No internal protections on this input are provided, but the user is encouraged to clamp or otherwise limit the voltage and current that may be applied to this input.

The default operating power corresponds to an input of 2.05 V. For maximum reliability it is recommended that power only be reduced, although if required it can be driven as high as 2.2 V (corresponding to a 10% boost in output power). The safest method of using this input is to pull the voltage down using an external resistor or potentiometer to ground. Applying a resistance to ground will create a voltage divide circuit between the external resistance and an internal resistance of 9.74 K to the 2.05 V reference.

Damage due to overdrive will not be covered under warranty. Use of this input will likely decrease the performance of the EM657 by bypassing its internal ultra-low noise voltage reference.

Warning: The PA input must never be shorted directly to V CC which would cause circuit malfunction or rapidly destroy the laser.

Temperature Adjust (TA)

The EM657 is designed to operate the laser chip at a constant temperature of 25°C holding the output frequency within a window of 5 GHz. However, some applications require coarse tuning of the output frequency via temperature. In these cases, the laser may be tuned using the TA input. Temperature deviations of more than a few degrees (50 GHz in laser frequency) may result in decreased stability and increases the likelihood of the laser experiencing a longitudinal mode-hop.

Use of this input carries the inherent potential of overdriving the TEC. The TA input is clamped to V CC through integrated protection diodes. If V ta is established before V CC these clamp diodes will conduct. The input current should always be limited to ≤3.5 mA to prevent destruction of the clamp diodes. The safest method of driving this input is with a tristate output whose output is current limited when active, maintained at high impedance until V CC is established, and whose output returns to high-impedance before V CC is removed.

The device warranty will not be honored for lasers with overdriven TECs. Use of this input also carries the likelihood of decreased frequency stability as it bypasses the internal ultra-low noise voltage.

Warning: The TA input must never be shorted directly to V CC or ground which would cause circuit malfunction or rapidly destroy the laser.

Grounding, DC to RF

Care must be taken with grounding, cabling, and connections due to the amount of current the module consumes. Make sure that the voltage on pins PA/TA reference ground as close to the EM657 as possible if either input is connected.

• DO NOT connect the cable shield to ground at both ends of the cable to avoid producing a ground loop.

The shield of the RF input 2.92 mm connector is connected to the laser anode and the center conductor to the laser cathode via the RF matching resistor. RF source should be AC coupled to the RF input. The RF and DC ground are at laser anode potential. An internal switching regulator is used to provide a negative reference to bias the laser.

The presence of this switching regulator allows for more convenient operation in systems where the RF and DC supply must have the same ground, but increases the noise level of the EM657 relative to the EM655 or EM650. The switching regulator noise causes a broadening of the laser line width to approximately 4MHz, and switching harmonics are visible in the low frequency RIN of the laser. This noise is no longer significant at frequencies greater than 200 MHz, and thus should not impact most high speed communication applications.

Startup Considerations

The EM657 consumes a considerable amount of current in the startup phase and when operating at temperature extremes. A voltage source plus cabling able to deliver the maximum specified current at no less than the minimum voltage is therefore needed. Current limiting below the specified maximum during the startup phase will result in an internally measured drive voltage lower than specified. This condition can result in permanent, non-warrantable
damage to the device.

Warning: If the user fails to sequence the supplies as described in the Power and Temperature Adjust sections of this document and Applications Note DS-7047, the device will immediately suffer non-warrantable damage or destruction.

Power Supply

This device requires between 4.6 V and 5.5 V as measured from the V CC to GND terminals. These voltages must be maintained for currents ranging from 0-3.5 A necessitating the use of short wires and/or large AWG wire.

Warning: Failure to supply sufficient voltage at the device terminals may result in excess current draw and permanent, non-warrantable damage. If the device draws 3.5 A for more than 10 seconds, turn off power and check for excessive wiring resistance or a baseplate temperature outside the operational range.

Additional Information

Be sure to check the website for the latest application information for this device. Application note DS-7047 (in Downloads) covers general usage of the EM657 along with information particular to tuning via temperature or chirp. If you plan to tune this device, we highly recommended that you read this application note.