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 30o and 10o, 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 30o).
Note: Parallel and perpendicular notation are specified relative to the junction plane of the laser diode.
. Perpendicular beam divergence from L780P010
, a style A laser diode
. 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 30o 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
= n sin(
/ 2) = sin(15) = 0.26 < NALens
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
. 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, = 10o, 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.
Lens Tube Mounting
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
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
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.