Features

• Small, Compact Size (1.75" x 1.75" x 1.66"
  (44.5 mm x 44.5 mm x 42.1 mm))
• Compatible with Pin Codes A, B, C, D, and E
• SM1 Thread and 30 mm Cage System Compatible

The LDM21 Miniature TEC-Cooled Laser Diode Mount measures half the size of our TCLDM9 mount and is capable of accepting both 5.6 mm and 9 mm laser diode packages. With an integrated thermal electric cooler element and a 10 kΩ thermistor, this mount keeps laser wavelengths stabilized by precisely holding the case temperature to within 0.002 °C. For more details on the operation and limitations of the temperature control, please see the TEC Operation tab above. The small size of the LDM21 makes this mount ideal for optical setups where space is limited.

Completely compatible with our extensive line of Laser Diode and TEC Controllers, this mount can operate with all style A, B, C, D, and E laser diode pin configurations. Please see the Pin Configurations tab for more info. The front side of the mount has a standard 1.035-40 thread, making it compatible with our SM1 Series of optical components and allowing for the addition of collimating or focusing optics. The back side of the mount accepts DB9 inputs from a laser current source and TEC controller.

Mounting collimating and other optics is easy using standard Thorlabs optomech parts. Step by step procedures for mounting optics and controlling the laser diode are found in the manual and the Pin Configurations, Electronic Control and LD Collimation tabs above. Contact tech support for more information.

Click to Enlarge

TEC Operation Limits

The LDM21 is intended to be operated at a fixed temperature setpoint of 25 °C but can be operated as low as 20 °C as long as the input power to the laser diode (LD current x LD voltage) does not exceed the values specified in the Safe Operating Area curve, as shown to the left, as a function of ambient temperature. It is not recommended that the LDM21 be driven with more than 600 mW of input power, set to a temperature below 20 °C, or operated in an ambient temperature above 35 °C. Operating the LDM21 outside of the safe operating area (the area under the curve in the figure to the right) will result in damage to the laser diode.

Laser Diode Pin Configurations

Thorlabs offers several different laser diodes in the visible and IR. The electrical connections for these diodes vary based on the internal circuitry of the diode. Many of the laser diodes feature a built-in monitor photodiode. Use the descriptions and schematics below, along with the appropriate style pin code (A - F), to properly power the laser diode.

The LDM21 TEC Laser Diode Mount is compatible with all Ø5.6 and Ø9 mm laser diodes with 3 pins in an A, B, or C style configuration shown in Figure 1. Style A, B, and C laser diodes contain both a laser diode and a photodiode for monitoring. These configurations feature independent control of the Laser Diode (LD) and Photodiode (PD) voltages, along with a common Ground (G) pin.

In addition, the mount is compatible with all of our currently available 4-pin laser diodes with a style D configuration, shown to the right in Figure 2. As with the style A, B, and C laser diodes, style D laser diodes also feature a laser diode and monitoring photodiode. Style D configurations, however, contain two ground pins instead of only one in those previous configurations. Please note that there is a difference between style D 4-pin and style F 4-pin (described below) configurations.

The LDM21 Mount is also compatible with style E laser diodes, which do not contain a corresponding photodiode. These laser diodes only contain a laser diode and ground pin, as shown to the right in Figure 2. The LDM21 is compatible with style E laser diodes where the LD pins are next to each other, as opposed to across from each other. This configuration is also discussed below.

The LDM21 mount is NOT compatible with style F 4-pin configurations. The style F pin configuration, though similar to the style D configuration in that it contains four pins, has a pin layout that prohibits use in the LDM21 mount. For more information, see the sections below or contact tech support.

Figure 3. TCLDM9 Standard Configurations

LDM21 Mounting Configuration

The LDM21 Laser Diode Mount has a standard 4-pin LD and PD mounting configuration. Both LD and PD connections can be made according to the diagram in Figure 3. Please note the orientation of the laser diode configuration (top/bottom and front/back) as it may vary between manufacturers, product lines, and/or configurations. The pin numbering convention on the laser diode manuals may differ as well.

The configuration shown to the right allows direct mounting of style A, B, C, D, and E configurations without any alteration to the mount. Some style E laser diodes require modification to the laser diode mount. Please see the next section for more details.

Style F laser diodes are incompatible with the LDM21 mount. The pin layout of the style F connectors place the LD and PD next to each other, instead of across from each other as shown in Figure 3 and described in detail below.

Style E Configuration

Style E configurations feature only a laser diode in the package; no photodiode is present. The LDM21 laser diode mount is compatible with some style E laser diode pin configurations if the LD and G pins of the laser diode are in a diagonal orientation (12 and 3 o'clock positions) as seen in Figures 2 and 3. If the LD and G pins are directly across from each other (i.e. in the LD and PD positions or 3 and 9 o'clock positions) the laser diode is not compatible with the LDM21. .

In order to drive a style E laser diode, please use Thorlabs' TCLDM9 Laser Diode Mount or contact tech support for further options. Note that modifications to the TCLDM9 will be necessary to drive style E laser diodes with this configuration.

Incompatible Style F Configurations

The LDM21 is not compatible with any style F connections, even though the configuration is very similar to the D style configuration.

The HL6548FG 660 nm, 90 mW laser diode features a style F configuration. A schematic showing the internal circuitry of the laser diode is shown in Figure 4. The incompatibility stems from the the arrangement of the PD and LD pins. In style F connectors the PD and LD pins are located diagonally from one another (9 and 12 o'clock positions, or any adjacent position). With this configuration, it is not possible to apply the correct voltage bias across both diodes simultaneously using the TCLDM9 laser diode mount.

For questions about compatibility of our style F laser diodes, or any other configuration, please contact tech support.

Electronic Assembly and Control

This guide will serve as an overview of the LDM21 electronic assembly and operation, as well as provide options for powering the laser diode and temperature controller. Full details of the assembly and operation of the LDM21 Mount can be found in the operating manual. The LDM21 can drive laser diodes with up to 1 A of current. The mount also provides a thermoelectric cooler (TEC) element with 5 W of cooling power. The TEC elements have a maximum current of 2.5 A at a maximum voltage of 1.8 V.

Laser Diode Controllers

Thorlabs offers a wide variety of laser diode controllers ranging from low power (low current and low voltage) to high power (high current and/or voltage) versions. Thorlabs also offers several dual laser diode current/temperature controllers and kits. Temperature controllers will be discussed in the next section. Laser diode current controllers should be chosen based on the actual laser diode used and the particular application.

Thorlabs' LDC2xxC series of controllers are suitable for use with a large majority of popular laser diodes. Thorlabs' LDC200CV is specifically designed to handle and safely operate Vertical Cavity Surface Emitting Lasers (VCSELs), while the LDC201CU provides users with an ultra-low noise current (<0.2 μA RMS) for stable operation of low power laser diodes. If your application requires the higher voltages typically necessary for driving blue and other short laser diodes, consider our LDC202C, LDC205C, or LDC210C controller. For driving higher power laser diodes, the LDC220C and LDC240C offer drive currents of 2 A and 4 A, respectively. Higher current (5 and 20 A), T-Cube-compatible, and rack mount controllers are also available. All of these controllers operate in a similar manor. Only the LDC2xxC series controllers will be discussed in more detail. Note that the higher power controllers are not meant to be used with the LDM21 as the maximum drive current of the LDM21 is 1A. Higher power mounts are available.

Prior to installing a laser diode in the mount, the pin configuration style must be determined and the mount properly configured to power the diode. There are two switches located under the front face plate of the mount that control the polarity of the laser diode and monitor photodiode, if present. To access these switches, remove the four hex screws holding the front plate to the mount. The switches are located above and below the LD mounting pins.

There are four pin configurations (A, B, C, and D), which are completely compatible with the LDM21 mount. The four pin styles are shown in Figure 1 along with the electronic schematic and switch settings. In these configurations, the laser diode houses a monitor photodiode tied to either the cathode or anode ground of the laser diode.

A fifth configuration, Style E, may be directly compatible with the mount depending on the orientation of the laser diode pins. These laser diodes do not have a monitor photodiode. See the Pin Configurations tab for more information on Style E compatibility.

Style F laser diodes are not compatible with the LDM21 mount. These laser diodes also feature a monitor photodiode; however, the pin layout prohibits the mount from powering the laser diode and photodiode simultaneously. Please see the Pin Configurations tab for more information.

Once the pin configuration is set, the controller can be connected to the mount via the DB9 male cable. The LDC2xxC series of controllers is preconfigured to interface directly with the mount. If a third-party controller is used, see the laser diode connector pin configuration in the operating manual to determine the proper connections.

TEC Controllers

Thorlabs also offers a wide variety of TEC controllers as stand-alone units, dual laser diode/ temperature controllers, and kits. The TED200C benchtop temperature controller is ideally suited to regulate the temperature of a laser diode mounted in the LDM21. This unit features a wide operating temperature range, 12 W of cooling, and high temperature stability. For more cooling power and even higher temperature stability, the TED4015 225 W temperature controller can be used in combination with the TCLDM9 mount.

The TEC elements in the LDM21 can be connected to a temperature controller via the DB9 female connection on the back of the unit. Adapter cables are available for temperature controllers with other connector types. For third-party controllers, please see the operating manual for pin layouts and descriptions. Follow the instructions for the TEC controller, paying careful attention not to overdrive the TEC elements in the mount.

LD Driver

9-Pin D-Type Female

TEC Controller

9-Pin D-Type Male

Choosing Collimation and Astigmatic Correction Optics for Your Laser Diode

Since the output of a laser diode is highly divergent, collimating optics are necessary. Due to their excellent ability to correct spherical aberration, aspheric lenses are the most commonly used optics when the desired collimated beam waist is between one and five millimeters. Choosing an appropriate aspheric lens for collimating a laser diode is essential, as the desired beam size and transmission range are dependent on the lens used. To calculate the beam size of a collimated laser diode, we first need to know its divergences.

The output of an edge-emitting laser diode is also highly astigmatic; the beam divergences will be different in the parallel and perpendicular directions, leading to an elliptical beam. By inserting anamorphic prism pairs or cylindrical lenses into the beam path after achieving collimation, it is possible to compensate for this ellipticity.

The divergences for a laser diode are typically specified as "Beam Divergence (FWHM) - Parallel" and "Beam Divergence (FWHM) - Perpendicular" for the two axes of the chip. There are variations from lot to lot of laser diodes, but using the typical divergence values should be adequate for most applications. A simple example will illustrate the key specifications to consider when choosing the correct optics for a given application.

Example: 785 nm, 25 mW Laser Diode (L780P010), Ø3 mm Desired Output Beam Diameter

Step 1: Collimating Emission

The specifications for the L780P010 laser diode indicate that the typical perpendicular and parallel beam divergences are 30^o and 10^o, respectively. The major (perpendicular) beam divergence is shown in Figure 1. The minor (parallel) beam divergence is shown in Figure 2. Because of this astigmatism or asymmetry in the two axes, an elliptical beam will form as the light diverges. To collect as much light as possible during the collimation process, consider the larger of these two divergence angles in any calculations (i.e., in this case use 30^o).

Note: Parallel and perpendicular notation are specified relative to the junction plane of the laser diode.

Figure 1. Perpendicular beam divergence from L780P010, a style A laser diode

Figure 2. Parallel beam divergence from L780P010, a style A laser diode

In the above schematics, LD denotes the laser diode, and are the beam diameters in the parallel and perpendicular orientations, respectively, and and are the divergence angle in the parallel and perpendicular orientations, respectively. Please note that the notch in Figures 1 and 2 can be used to determine the orientation of the laser diode within the package. Laser diodes are typically oriented parallel to the notch; however there are many exceptions, especially for different laser diode packagings. Care should be taken to properly orient the laser diode and laser diode emission.

To calculate the focal length needed to acheive a Ø3 mm collimated beam diameter, we can use:

where is the focal length that produces the desired perpendicular beam diameter,. The focal length of the lens needed to collimate a 30^o diverging beam into a Ø3 mm collimated beam is = 5.6 mm.

Equation 1 yields the focal length to achieve our desired major (perpendicular) axis diameter. Use this to then select an aspheric lens with a focal length that most closely matches the focal length given by the equation. Please note that the diameter of the lens must be larger than your desired major axis beam diameter.

Thorlabs offers a large selection of aspheric lenses. For this application, the ideal lens is an molded glass aspheric lens with an antireflection coating for the 600-1050 nm range and a focal length near 5.6 mm. The C170TME-B (mounted) or 352170-B (unmounted) aspheric lens has a focal length of 6.16 mm. Next, check to see if the numerical aperture (NA) of the diode is smaller than the NA of the lens so that the light emitted from the laser diode is not clipped by the lens using the equation

NA_Diode = n sin( / 2) = sin(15) = 0.26 < NA_Lens = 0.30

Here, n is the refractive index (in air here so n = 1). Solving Eq. 1 again with your actual focal length and major axis divergence angle yields the actual major axis beam diameter, = 3.3 mm.

Step 2: Correcting Astigmatism

Figure 3. Anamorphic Prism Pair and optic trace for an ellipse to round beam.

Emission from an edge-emitting laser diode is astigmatic (asymmetric with respect to two different axes), as shown in Figures 1 and 2. To correct for this and produce a circular beam, the minor axis diameter, , can be magnified using anamorphic prism pairs or cylindrical lenses after collimation. Figure 3 shows an anamorphic prism pair magnifying an elliptical beam minor axis to produce the desired symmetric beam.

To determine what magnification of the minor axis is needed to produce a round beam, solve Eq. 1 using the focal length from the aspheric lens, = 6.16 mm, and minor axis divergence for the laser diode, = 10^o, instead of the major axis divergence. This results in a minor axis diameter, = 1.1 mm. Comparing and , we see that a 3X magnification is necessary in the minor beam axis. This 3X magnification can be acheived using a PS879-B Mounted Anamorphic Prism Pair.

Step 3: Assembling the Laser Diode System

Assembly of the laser diode system begins by removing the front plate of the TCLDM9 and installing the laser diode according to the operating manual. The installation of a Ø5.6 mm laser diode is shown in Figures 4 - 8 below. The TCLDM9 can hold Ø9 mm and Ø5.6 mm laser diodes, utilizing the correct retainer ring. Once the diode and front plate of the TCLDM9 are installed, an S1TMxx adapter can be installed using the SM1 threads on the front of the adapter. Cage mounting is also possible using the 30 mm cage threads on the mount.

Figure 4. TCLDM9 LD Mount

Figure 5. Front Plate Removal

Figure 6. LD Retainer Removal

Figure 7. LD Installation

Figure 8. Install LD Retainer

Lens Tube Mounting

Figure 9. C170TME-B Mounted Aspheric Lens and S1TM08 Adapter

Figure 11. C170TME-B Mounted Aspheric Lens with S05TM8 and SM1A6T Adapters

Figure 10. TCLDM9 with C170TME-B Lens and S1TM08 Adapter

Figure 12. TCLDM9 with C170TME-B Lens and S05TM08 and SM1A6T Adapters

Figure 13. 352170-B Aspheric Lens, LMRA8 and SM1A6T Adapters

Figure 14. TCLDM9 with C220TME-B Mounted Aspheric Lens, S1TM09 Adapter, and SM1Z Adjuster
(Note: One cage rode has been removed for viewing purposes)

For mounted aspheric lenses with focal lengths less than 4 mm, the S1TMxx adapters can be used. These adapters are ideal for this application, as the aspheric lens mounts flush to the inside surface of the adapter and the outer threading has a rubber O-ring to keep the adapter from rotating in the TCLDM9's front plate.

A mounted aspheric lens, C170TME-B, with a focal length of 6.16 mm mounted in a S1TM08 adapter is shown on the TCLDM9 mount in Figure 9. In this configuration, the maximum focal length is dictated by the total translation of the adapter away from the mount, shown in Figure 10.

For mounted aspheric lenses with focal lengths greater than 4 mm and less than 10 mm, we recommend using an S05TMxx adapter in conjunction with a SM05 to SM1 adapter. The S05TMxx can then be mounted in an SM1A6T SM1-to-SM05 adapter and screwed onto the mount. The SM1A6T adapter can mount aspheric lenses with focal lengths up to 10 mm. For longer focal lengths, contact tech support or see the Cage Assembly Mounting instructions below.

In the above example, the C170TME-B mounted lens features M8 x 0.5 threading, thus requiring the S05TM08-threaded adapter. The S05TM08 M8-to-SM05 adapter can be mounted in the TCLDM9 laser diode mount using the SM1A6T SM1-to-SM05 adapter. This arrangement is shown in Figure 11. The correct distance between the laser diode and lens can be acheived by adjusting both the S05TM08 and the SM1A6T adapters. This optic arrangement is shown with the TCLDM9 in Figure 12.

Unmounted aspheres can be epoxied to an LMRAxx adapter, which can then be mounted in an SM1A6T SM1-to-SM05 adapter. The adapter's SM1 threading can be used to attach the lens/mount/adapter assembly to the TCLDM9 front pate. The SM1A6T adapter has a translation range of 10 mm in the TCLDM9. This translation range covers almost the entire focal length range of our aspheric lenses.

If the 352170-B unmounted ashperic lens is used, it must be epoxied to the LMRA8 adapter prior to mounting it in the SM1A6T SM1-to-SM05 adapter. Again, adjustment of the aspheric lens distance from the laser diodes can be made by translating both the LMRA8 and SM1A6T adapters in the TCLDM9. This arrangement is shown in Figure 13.

Cage Assembly Mounting

Mounted and unmounted aspheric lenses with focal lengths greater than 8 mm can be cage mounted using our 30 mm cage system. Cage rods attach directly to the front plate of the TCLDM9 mount. The CP02 SM1-Threaded, 30 mm Cage Plate can house the the S1TMxx adapter with mounted aspheric lens or the SM1A6T adapter with unmounted aspheric lens epoxied to an LMRAxx adapter.

For fine adjustment of the aspheric lens, the SM1Z translator for cage systems can be used in lieu of the CP02 cage mount. This z-translator provides 1.5 mm of travel and incremental movements of 1 μm. For larger translational adjustments, the CT1 1/2" Travel Translator can be used. The SM1Z cage-mounted translation adjuster with mounted asphere is shown in Figure 14.

Aspheric lenses can also be mounted on an extended thread adapter such as the E09RMS. This extended adapter allows the asphere to be positioned as close as needed to the laser diode. The SM1A3 will allow the E09RMS to be mounted in the SM1 threaded cage mounted translator.

Correcting Astigmatism in the Output Beam

Figure 16. Cylindrical Lens Pair

Figure 15. Mounted Anamorphic Prism Pair

The astigmatic output of the laser diode can be corrected using either anamorphic prisms or cylindrical lenses. As determined in the example above, a 3X mounted anamorphic prism pair (i.e. PS879-B ) was needed to produce a round beam profile. Unmounted prisms may be used as well.

The PS879-B Mounted Anamorphic Prism Pair features SM05 threading on the output end or may be mounted inside an SM1 Lens tube. A mounted anamorphic prism pair is shown in Figure 15.

A pair of cylindrical lenses, shown in Figure 16, may also be used to correct for astigmatism in the output of the laser diode. A plano-convex and plano-concave cylindrical lens pair may be used to compress the perpendicular beam waist or expand the parallel beam waist. The lens pair can be mounted within the same SM1 lens tube, which can be attached to the TCLDM9 mount, or independently in seperate optic mounts.

Multimode Fiber Coupling of a Laser Diode

Figure 18. Fiber Coupling Using a ST1XY-S Adjuster and LDM21 Laser Diode Mount

Figure 17. Fiber Coupling Arrangement using an FC Fiber, F240FC-B Coupler,and AD12F Adapter

The TCLDM9 makes an ideal mount to build a device to couple light from a laser diode into a multimode fiber. Thorlabs' offers a wide variety of premounted and prealigned optics for coupling light into and out of optical fibers. A basic light coupling device is shown in Figure 17. Here, a fixed focus, aspheric collimation/coupling package, F240FC-B, is used as the coupling device into an FC connectorized fiber. The collimator can be mounted in an SM1 lens tube system via the AD12F adapter.

Note: For maximum coupling efficiency, the collimated light from the laser diode should fill the clear aperture of the coupler. In addition, the NA of the coupler must be smaller than the NA of the fiber.

An ST1XY-S XY Translator can be used to couple laser diode emission into the mounted fiber. The translator is 30 mm cage and SM1 lens tube compatible. This simple arrangement is shown in Figure 18 for the LDM21 Laser Diode Mount. For more precise alignment, a differential drive ST1XY-D XY Translator may be used to align the XY axis and SM1Z adjuster to align the Z distance/axis of the aspheric collimation lens.

Note: The above coupling works well for relatively large NA, large core multimode fiber, however will exhibit high losses (low coupling efficiencies) for single mode fiber because of the low NA and small mode field diameter inherent to these types of fiber. To couple light into a single mode fiber, we recommend using an adjustment mirror and a Fiber Launch, such as the MAX350D Professional Fiber Launch System. The fiber launch uses an objective to focus the light into a single mode fiber mounted to the bench.