Quantum Cascade Lasers (QCLs): Distributed Feedback, Two-Tab C-Mount

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Features

• Single-Wavelength Distributed Feedback Quantum Cascade Lasers (QCLs)
• Typical Output Power from 40 to 100 mW, Depending on Device
• Center Wavelengths from 4.00 µm to 11.00 μm (Wavenumbers Between 2500 cm^-1 and 909 cm^-1)
• Compact Two-Tab C-Mount Package: 6.4 mm x 4.3 mm x 7.9 mm (L x W x H)
• Electrically Isolated from C-Mount
• Custom Wavelengths and Mounts Also Available

Thorlabs' Distributed Feedback Quantum Cascade Lasers (DFB QCLs) emit at a well defined center wavelength and provide single transverse mode operation. By tuning the input current and operating temperature, the output frequency can be tuned over a narrow range between 1 cm^-1 and 5 cm^-1. These lasers are ideal for chemical sensing (see the Spectroscopy tab), optical communications, and other applications. Thorlabs also manufactures Fabry-Perot Quantum Cascade Lasers and Interband Cascade Lasers, which exhibit broadband emission.

Before shipment, the output spectrum, power, and L-I-V curve are measured for each serial-numbered device by an automated test station. These measurements are available below and are also included on a data sheet with the laser. These QCLs are specified for CW output. While pulsed output is possible, this application prohibits current tuning, and performance is not guaranteed. Each QCL has an uncoated back facet and an uncoated or AR-coated front facet (see the Appearance tab for details). Please note that some optical power is emitted through the rear facet; this output is not usable in applications.

Packages
Each DFB quantum cascade laser is mounted on a two-tab C-mount that provides high thermal conductivity and can be secured using a 2-56 or M2 screw with the counterbored Ø2.4 mm (Ø0.09") through hole. As measured from the bottom of the C-mount, the emission height is either 7.15 mm or 7.39 mm depending on the chosen QCL. Click on a laser's blue info icon () and view the Drawing tab to find the laser's emission height. QCLs are electrically isolated from their C-mounts. Please see the Handling tab for more tips and information for handling these laser packages.

Mounts, Drivers, and Temperature Control
We generally recommend the LDMC20 C-Mount Laser Mount and ITC4002QCL or ITC4005QCL Dual Current / Temperature Controller for use with our distributed feedback QCLs. This device combination includes all the necessary components to mount, drive, and thermally regulate a two-tab C-mount laser. Other compatible current and temperature controllers are listed in the Drivers tab.

If designing your own mounting solution, note that due to these lasers' heat loads, we recommend that they be mounted in a thermally conductive housing to prevent heat buildup. Heat loads for distributed feedback QCLs can be up to 12.6 W (see the Handling tab for additional information).

The typical operating voltage of distributed feedback quantum cascade lasers is 9 - 14 V. These lasers do not have built-in monitor photodiodes and therefore cannot be operated in constant power mode.

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Available Wavelengths for Custom DFB Lasers

DFB QCLs at Custom Wavelengths

Thorlabs manufactures custom and OEM quantum cascade lasers in high volumes. We maintain chip inventory from 3 µm to 12 µm at our Jessup, Maryland laser manufacturing facility, and can deliver DFB lasers with custom center wavelengths that are qualified to a user-defined wavelength precision.

More details are available on the Custom & OEM Lasers tab. To inquire about pricing and availability, please contact us. A semiconductor specialist will contact you within 24 hours or the next business day.

Use the tables below to select a compatible controller for our MIR lasers. The first table lists the controllers with which a particular MIR laser is compatible, and the second table contains selected information on each controller; complete information on each controller is available in its full web presentation. We particularly recommend our ITC4002QCL and ITC4005QCL controllers, which have high compliance voltages of 17 V and 20 V, respectively. Together, these drivers support the current and voltage requirements of our entire line of Mid-IR Lasers. To get L-I-V and spectral measurements of a specific, serial-numbered device, click "Choose Item" next to the part number below, then click on the Docs Icon next to the serial number of the device.

Laser Mount Compatibility
Thorlabs' LDMC20 C-Mount Laser Mount ships with current and TEC cables for the LDC4005, ITC4001, ITC4002QCL, ITC4005, and ITC4005QCL controllers. If designing your own mounting solution, note that due to these lasers' heat loads, we recommend that they be secured in a thermally conductive housing to prevent heat buildup. Heat loads for distributed feedback QCLs can be up to 12.6 W.

Laser and Controller Compatibility

Controller Selection Guide

Handling Two-Tab C-Mount Lasers

Proper precautions must be taken when handling and using two-tab C-mount lasers. Otherwise, permanent damage to the device will occur. Members of our Technical Support staff are available to discuss possible operation issues.

Avoid Static
Since these lasers are sensitive to electrostatic shock, they should always be handled using standard static avoidance practices.

Avoid Dust and Other Particulates
Unlike TO can and butterfly packages, the laser chip of a two-tab C-mount laser is exposed to air; hence, there is no protection for the delicate laser chip. Contamination of the laser facets must be avoided. Do not blow on the laser or expose it to smoke, dust, oils, or adhesive films. The laser facet is particularly sensitive to dust accumulation. During standard operation, dust can burn onto this facet, which will lead to premature degradation of the laser. If operating a two-tab C-mount laser for long periods of time outside a cleanroom, it should be sealed in a container to prevent dust accumulation.

Use a Current Source Specifically Designed for Lasers
These lasers should always be used with a high-quality constant current driver specifically designed for use with lasers, such as any current controller listed in the Drivers tab. Lab-grade power supplies will not provide the low current noise required for stable operation, nor will they prevent current spikes that result in immediate and permanent damage.

Thermally Regulate the Laser
Temperature regulation is required to operate the laser for any amount of time. The temperature regulation apparatus should be rated to dissipate the maximum heat load that can be drawn by the laser. For our quantum cascade lasers, this value can be up to 18 W. The LDMC20 C-Mount Laser Mount, which is compatible with our two-tab C-mount lasers, is rated for >20 W of heat dissipation.

The back face of the C-mount package is machined flat to make proper thermal contact with a heat sink. Ideally, the heat sink will be actively regulated to ensure proper heat conduction. A Thermoelectric Cooler (TEC) is well suited for this task and can easily be incorporated into any standard PID controller.

A fan may serve to move the heat away from the TEC and prevent thermal runaway. However, the fan should not blow air on or at the laser itself. Water cooling methods may also be employed for temperature regulation. Do not use thermal grease with this package, as it can creep, eventually contaminating the laser facet. Pyrolytic graphite is an acceptable alternatives to thermal grease for these packages. Solder can also be used to thermally regulate two-tab C-mount lasers, although controlling the thermal resistance at the interface is important for best results.

For assistance in picking a suitable temperature controller for your application please contact Tech Support.

Carefully Make Electrical Connections
When making electrical connections, care must be taken. The flux fumes created by soldering can cause laser damage, so care must be taken to avoid this. Solder can be avoided entirely for two-tab C-mount lasers by using the LDMC20 C-Mount Laser Mount. If soldering to the tabs, solder with the C-mount already attached to a heat sink to avoid unnecessary heating of the laser chip.

Minimize Physical Handling
As any interaction with the package carries the risk of contamination and damage, any movement of the laser should be planned in advance and carefully carried out. It is important to avoid mechanical shocks. Dropping the laser package from any height can cause the unit to permanently fail.

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C-Mount Laser with Uncoated Front Facet (Left) and AR-Coated Front Facet (Right)

Appearance

In order to ensure that the finished lasers meet their target specifications, they are qualified by an automated test station before shipment. The test station measures the output spectrum, optical power, and L-I-V curve, which are included on a data sheet that ships with each serial-numbered laser. These measurements are also available below by clicking "Choose Item" and then clicking on the red Docs icon () that appears.

Because every laser is a unique device, certain units require the front facet to receive an anti-reflection (AR) coating to reach the specified output power. As a result, finished units will have varying appearances, as shown by the photo to the right. These cosmetic differences do not affect the performance of the laser when it is operated within its specified parameters. All shipped units have been burned in and have passed our rigorous optical performance, reliability, and environmental testing.

Choosing a Collimating Lens

Since the output of our MIR lasers is highly divergent, collimating optics are necessary. Aspheric lenses, which are corrected for spherical aberration, are commonly chosen when the desired beam diameter is between 1 - 5 mm. The simple example below illustrates the key specifications to consider when choosing the correct lens for a given application.

The following example uses our previous generation 3.8 µm Interband Cascade Laser.

Key Specifications

• Center Wavelength: 3.80 µm
• Parallel Beam Divergence Angle: 40°
• Perpendicular Beam Divergence Angle: 60°
• Desired Collimated Beam Diameter: 4 mm (Major Axis)

The specifications for the laser indicate that the typical parallel and perpendicular FWHM divergences are 40° and 60°, respectively. Therefore, as the light propagates, an elliptical beam will result. To collect as much light as possible during the collimation process, consider the larger of these two divergence angles in your calculations (in this case, 60°).

θ = Divergence Angle
Ø = Beam Diameter

Using the information above, the focal length needed to obtain the desired beam diameter can be calculated:

This information allows the appropriate collimating lens to be selected. Thorlabs offers a large selection of black diamond aspheric lenses for the mid-IR spectral range. Since this laser emits at 3.80 µm, the best AR coating is our -E coating, which provides R_avg < 0.6% per surface from 3 to 5 µm. The lenses with focal lengths closest to the calculated value of 3.46 mm are our 390036-E (unmounted) or C036TME-E (mounted) Molded Aspheric Lenses, which have f = 4.00 mm. Plugging this focal length back into the equation shown above gives a final beam diameter of 4.62 mm along the major axis.

Next, we verify that the numerical aperture (NA) of the lens is larger than the NA of the laser:

NA_Lens = 0.56

NA_Laser ~ sin (30°) = 0.5

NA_Lens > NA_Laser

Since NA_Lens > NA_Laser, the 390036-E or C036TME-E lenses will give acceptable beam quality. However, by using the FWHM beam diameter, we have not accounted for a significant fraction of the beam power. A better practice is to use the 1/e² beam diameter. For a Gaussian beam profile, the 1/e² beam diameter is approximately equal to 1.7X the FWHM diameter. The 1/e² beam diameter is therefore a more conservative estimate of the beam size, containing more of the laser's intensity. Using this value significantly reduces far-field diffraction (since less of the incident light is clipped) and increases the power delivered after the lens.

A good rule of thumb is to pick a lens with an NA of twice the NA of the laser diode. For example, either the 390037-E or the C037TME-E could be used as these lenses each have an NA of 0.85, which a little less than twice that of our IF3800CM2 laser (NA 0.5). Compared to the first set of lenses we identified, these have a shorter focal length of 1.873 mm, resulting in a smaller final beam diameter of 2.16 mm.

Beam Profile Characterization of a Mid-IR Laser

Because quantum cascade lasers (QCLs) and interband cascade lasers (ICLs) have intrinsically large divergence angles, it is necessary to install collimating optics in front of the laser face, as shown in the Collimation tab. We are frequently asked what beam quality can be reasonably expected once the beam has been collimated. This tab presents an M² measurement we performed using our previous generation 3.80 µm Interband Cascade Laser.

For this system, we obtained M² = 1.2 ± 0.08 in the parallel direction and M² = 1.3 ± 0.2 in the perpendicular direction. While this is just one example, we believe these results to be representative of well-collimated mid-IR lasers manufactured by Thorlabs, as corroborated by supplementary measurements we have performed in-house.

Experimental Setup

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Pyroelectric Camera Upstream of Focus

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Pyroelectric Camera Downstream of Focus

The apparatus we used to determine M² is shown schematically in the figure above. In order to ensure that our results were rigorous, all data acquisition and analysis were consistent with the ISO11146 standard.

The previous generation Interband Cascade Laser used for this measurement emitted CW laser light with a center wavelength of 3.781 µm. Our LDMC20 temperature-stabilized mount held the laser's temperature at 25 °C. The output beam was collimated by a C037TME-E lens located immediately downstream of the laser face. This lens was selected because of its large NA of 0.85 (which helped maximize collection of the emitted light) and because of its AR coating (R_avg < 0.6% per surface from 3 µm to 5 µm). We measured 10 mW of output power after the lens.

A pyroelectric camera (Spiricon Pyrocam IV) with 80 µm square pixels was scanned along the beam propagation direction, and the beam width was measured along the parallel and perpendicular directions using the second-order moment (D4σ) definition. Hyperbolas were fit to the beam width to extract M² for each direction. The camera's internal chopper was triggered at 50 Hz since the pyroelectric effect is sensitive to changes in temperature rather than absolute temperature differences. A ZnSe window was present in front of the detector array to help minimize visible light contributions to the signal.

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D4σ Beam Width of Collimated IF3800CM2 Laser

Data Analysis
Presented to the right are the second-order moment (D4σ) beam widths for the parallel and perpendicular directions as a function of distance from the laser face (denoted as z). Along the parallel direction, we obtained a minimum beam width of 1.5 mm, while along the perpendicular direction, we obtained a minimum beam width of 1.3 mm. The spatial profiles we observed at the two minimum beam width positions, as obtained by the pyroelectric camera, are shown below.

The divergence of the beam can be described by a hyperbola, as written in Equation 1:

(Eq. 1)

In order to obtain the hyperbola coefficients a, b, and c for the parallel and perpendicular directions, we fit the discrete beam width measurements along each direction to hyperbolas, as shown in the graph to the right. These coefficients were substituted into Equation 2 (taking λ = 3.781 µm) to yield M².

(Eq. 2)

The hyperbola coefficients and M² values derived by this method are listed in the table below.

The 0.85 NA of the collimating lens we used is the largest NA of any lens for this wavelength range that is offered in our catalog. Despite this large NA, we observed lobes in the far field (shown by the figure below) that are consistent with clipping of the laser-emitted light. An ideal measurement would not contain these artifacts.

As shown by the graph above and to the right, we observed significant astigmatism in the collimated beam: the beam waist of the parallel direction occurred around z = 300 mm, while the beam waist of the perpendicular direction occurred around z = 600 mm. This astigmatism corresponds closely to what is expected for this laser, given that the IF3800CM2 laser is specified with a parallel FWHM beam divergence of 40° and a perpendicular FWHM beam divergence of 60°.

Beam Profile at Beam Waist of Parallel Direction
(Each Pixel is 80 µm Square)

Beam Profile at Beam Waist of Perpendicular Direction
(Each Pixel is 80 µm Square)

Gas-Phase Spectroscopy Using Distributed Feedback Lasers

Distributed Feedback Quantum Cascade Lasers (DFB QCLs) offer many attractive features for spectroscopy. They emit at a single wavelength within the mid-IR, where many gaseous species characteristically absorb. Moreover, their emission wavelength is easily tuned (typical tuning range: 1 - 5 cm^-1) by changing the drive current and operating temperature of the laser, making them ideal for isolating narrow gas absorption lines. Finally, they offer relatively high output power (typically 40 - 120 mW at rollover current), helping improve measurement sensitivity.

Thorlabs' DFB QCLs emit at wavelengths that range from 4.00 to 11.00 µm (2500 cm^-1 to 909 cm^-1). If we do not stock the wavelength required for your application, custom wavelengths are available by contacting Tech Support.

The tuning range of individual DFB QCLs depends greatly on the actual laser device. Each DFB QCL is a unique device with its own threshold current, rollover current, and spectrum. With typical lasers, it is usually preferable to operate the laser at or near the rollover current, since the output power is lowest at threshold and highest at rollover. On the other hand, the wavelength of DFB QCLs changes as a function of the current, so operating at the rollover current is not always possible in spectroscopy measurements, which require specific wavelengths. (It is important to note that the output power is not constant over the entire tuning range.)

DFB QCLs and ICLs are ideal for use in photoacoustic spectroscopy, a technique based on the photoacoustic effect that is able to accurately detect trace gas concentrations for a wide variety of applications. Thorlabs offers an Acoustic Detection Module that can be used with our DFB QCLs and ICLs to build custom QEPAS sensors that target the absorption of a specific gas. We also offer a Quartz-Enhanced Photoacoustic Sensor that targets a methane absorption line to detect trace amounts of methane in a gas.

Tuning Example
To demonstrate DFB QCLs' tunability, we measured the center wavelength of a previous-generation QD4580CM1 laser as a function of drive current, from threshold to near rollover, at 15 °C and 25 °C. Over the entire temperature and drive current range, we obtained center wavelengths from 4.568 µm to 4.586 µm (2189.14 cm^-1 to 2180.77 cm^-1), spanning a range of 18 nm (8.37 cm^-1), with output power ranging from 3.2 mW (at threshold current) to 39.1 mW (at near-rollover current). Since the laser is capable of operating at 35 °C, even broader wavelength tuning is also achievable.

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DFB QCL Center Frequency as Function of Temperature and Drive Current

ECL, DFB, VHG-Stabilized, and DBR Single-Frequency Lasers

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Figure 1: ECL Lasers have a Grating Outside of the Gain Chip

A wide variety of applications require tunable single-frequency operation of a laser system. In the world of diode lasers, there are currently four main configurations to obtain a single-frequency output: external cavity laser (ECL), distributed feedback (DFB), volume holographic grating (VHG), and distributed Bragg reflector (DBR). All four are capable of single-frequency output through the utilization of grating feedback. However, each type of laser uses a different grating feedback configuration, which influences performance characteristics such as output power, tuning range, and side mode suppression ratio (SMSR). We discuss below some of the main differences between these four types of single-frequency diode lasers.

External Cavity Laser
The External Cavity Laser (ECL) is a versatile configuration that is compatible with most standard free space diode lasers. This means that the ECL can be used at a variety of wavelengths, dependent upon the internal laser diode gain element. A lens collimates the output of the diode, which is then incident upon a grating (see Figure 1). The grating provides optical feedback and is used to select the stabilized output wavelength. With proper optical design, the external cavity allows only a single longitudinal mode to lase, providing single-frequency laser output with high side mode suppression ratio (SMSR > 45 dB).

One of the main advantages of the ECL is that the relatively long cavity provides extremely narrow linewidths (<1 MHz). Additionally, since it can incorporate a variety of laser diodes, it remains one of the few configurations that can provide narrow linewidth emission at blue or red wavelengths. The ECL can have a large tuning range (>100 nm) but is often prone to mode hops, which are very dependent on the ECL's mechanical design as well as the quality of the antireflection (AR) coating on the laser diode.

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Figure 2: DFB Lasers Have a Bragg Reflector Along the Length of the Active Gain Medium

Distributed Feedback Laser
The Distributed Feedback (DFB) Laser (available in NIR and MIR) incorporates the grating within the laser diode structure itself (see Figure 2). This corrugated periodic structure coupled closely to the active region acts as a Bragg reflector, selecting a single longitudinal mode as the lasing mode. If the active region has enough gain at frequencies near the Bragg frequency, an end reflector is unnecessary, relying instead upon the Bragg reflector for all optical feedback and mode selection. Due to this “built-in” selection, a DFB can achieve single-frequency operation over broad temperature and current ranges. To aid in mode selection and improve manufacturing yield, DFB lasers often utilize a phase shift section within the diode structure as well.

The lasing wavelength for a DFB is approximately equal to the Bragg wavelength:

where λ is the wavelength, n_eff is the effective refractive index, and Λ is the grating period. By changing the effective index, the lasing wavelength can be tuned. This is accomplished through temperature and current tuning of the DFB.

The DFB has a relatively narrow tuning range: about 2 nm at 850 nm, about 4 nm at 1550 nm, or at least 1 cm^-1 in the mid-IR (4.00 - 11.00 µm). However, over this tuning range, the DFB can achieve single-frequency operation, which means that this is a continuous tuning range without mode hops. Because of this feature, DFBs have become a popular and majority choice for real-world applications such as telecom and sensors. Since the cavity length of a DFB is rather short, the linewidths are typically in the 1 MHz to 10 MHz range. Additionally, the close coupling between the grating structure and the active region results in lower maximum output power compared to ECL and DBR lasers.

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Figure 3: VHG Lasers have a Volume Holographic Grating Outside of the Active Gain Medium

Volume-Holographic-Grating-Stabilized Laser
A Volume-Holographic-Grating-(VHG)-Stabilized Laser also uses a Bragg reflector, but in this case a transmission grating is placed in front of the laser diode output (see Figure 3). Since the grating is not part of the laser diode structure, it can be thermally decoupled from the laser diode, improving the wavelength stability of the device. The grating typically consists of a piece of photorefractive material (typically glass) which has a periodic variation in the index of refraction. Only the wavelength of light that satisfies the Bragg condition for the grating is reflected back into the laser cavity, which results in a laser with extremely wavelength-stable emission. A VHG-Stabilized laser can produce output with a similar linewidth to a DFB laser at higher powers that is wavelength-locked over a wide range of currents and temperatures.

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Figure 4: DBR Lasers have a Bragg Reflector Outside of the Active Gain Medium

Distributed Bragg Reflector Laser
Similar to DFBs, Distributed Bragg Reflector (DBR) lasers incorporate an internal grating structure. However, whereas DFB lasers incorporate the grating structure continuously along the active region (gain region), DBR lasers place the grating structure(s) outside this region (see Figure 4). In general a DBR can incorporate various regions not typically found in a DFB that yield greater control and tuning range. For instance, a multiple-electrode DBR laser can include a phase-controlled region that allows the user to independently tune the phase apart from the grating period and laser diode current. When utilized together, the DBR can provide single-frequency operation over a broad tuning range. For example, high end sample-grating DBR lasers can have a tuning range as large as 30 - 40 nm. Unlike the DFB, the output is not mode hop free; hence, careful control of all inputs and temperature must be maintained.

In contrast to the complicated control structure for the multiple-electrode DBR, a simplified version of the DBR is engineered with just one electrode. This single-electrode DBR eliminates the complications of grating and phase control at the cost of tuning range. For this architecture type, the tuning range is similar to a DFB laser but will mode hop as a function of the applied current and temperature. Despite the disadvantage of mode hops, the single-electrode DBR does provide some advantages over its DFB cousin, namely higher output power because the grating is not continuous along the length of the device. Both DBR and DFB lasers have similar laser linewidths. Currently, Thorlabs offers only single-electrode DBR lasers.

Conclusion
ECL, DFB, VHG, and DBR laser diodes provide single-frequency operation over their designed tuning range. The ECL can be designed for a larger selection of wavelengths than either the DFB or DBR. While prone to mode hops, it also provides the narrowest linewidth (<1 MHz) of the three choices. In appropriately designed instruments, ECLs can also provide extremely broad tuning ranges (>100 nm).

The DFB laser is the most stable single-frequency, tunable laser of the four. It can provide mode-hop-free performance over its entire tuning range, making it one of the most popular forms of single-frequency laser for much of industry. It has the lowest output power due to inherent properties of the continuous grating feedback structure.

The VHG laser provides the most stable wavelength performance over a range of temperatures and currents and can provide higher powers than are typical in DFB lasers. This stability makes it excellent for use in OEM applications.

The single-electrode DBR laser provides similar linewidth and tuning range as the DFB (<5 nm). However, the single-electrode DBR will have periodic mode hops in its tuning curve.