Frequently Asked Questions
Quantum Cascade Lasers (QCL)
- What is a QCL?
A Quantum Cascade Laser (QCL) is a semiconductor laser that emits highly coherent radiation in the mid- to long-wave infrared region of the spectrum. QCLs are not diode lasers, but rather unipolar semiconductor devices consisting of hundreds of epitaxial grown layers forming a large number of quantum wells in the conduction band of the device and engineered to enable a cascade of photons emitted for each injected electron. QCLs generate light in the 4 µm to 25 µm region of the electromagnetic spectrum. For a more in-depth summary, please visit: http://en.wikipedia.org/wiki/Quantum_cascade_laser
- What is an ECqcL™?
An External Cavity Quantum Cascade Laser (ECqcL™) is a semiconductor laser source patented by Daylight Solutions, which integrates quantum cascade gain media into an external cavity having wavelength dependent feedback. ECqcLs™ are available as either precision fixed-wavelength sources, or as broadly tunable lasers. A tunable ECqcL™ can tune across the entire gain profile of the QC chip, allowing for tunability of 10% to 25% of the center wavelength.
- What is an ECicL™?
An External Cavity Interban Cascade Laser (ECicL™) is a laser source patented by Daylight Solutions, which integrates interband cascade gain media into an external cavity providing wavelength-dependent feedback. ECicLs™ operate from 3-4 µm and are available as either precision fixed-wavelength sources, or as broadly tunable lasers. A tunable ECicL™ can tune across the entire gain profile of the QC chip, allowing for tunability greater than 10% of the center wavelength.
- What is the basic difference between a DFB and FP QCL?
A Distributed Feed-Back (DFB) QCL has a diffraction grating grown into the active region of the semiconductor laser. This grating provides narrow band optical feedback, distributed along the length of the waveguide, eliminating the need for discrete mirrors to form an optical cavity. DFBs allow for semi-stable fixed wavelength sources. A Fabry-Perot (FP) QCL uses the cleaved facet ends of the chip to form the 2 reflective surfaces of the laser cavity. Since all wavelengths are reflected equally in a FP QCL, all wavelengths in the gain profile of the chip are available for lasing.
- What is the mechanism for tuning the ECqcL™ wavelength?
Daylight Solutions' tunable laser sources utilize disc drive technology to control a wavelength-selective diffraction grating within an external cavity configuration. As the grating angle and position is adjusted, the wavelength-dependent feedback into the gain media is varied (tuned). This implementation allows tunability across the entire gain band of the gain media.
- Is the External Cavity a Littman or Littrow configuration?
Daylight Solutions' ECqcLs™ are manufactured in a Littrow configuration, allowing independent control of the output power and the wavelength-dependent feedback from the external cavity.
- At what temperature do ECqcLs™ operate?
All Daylight Solution laser sources utilize QC gain media controlled to room temperature and integrated into systems that can operate over a wide temperature range. For example, Daylight Solutions' high-power laser systems for defense applications operate from -55°C to +71°C.
- Is liquid cooling required?
Liquid N2 is NOT required for cooling. Daylight Solutions' Continuous Wave (CW) laser sources for scientific applications utilize an external water chiller to remove excess heat from the base of the TEC that is controlling the QCL. The heat capacity is less than 25W so most small water chillers will work.
- How many laser platforms are there?
Daylight Solutions offers four scientific laser platforms. Additional systems are available for multi-wavelength applications or sensor applications. Contact Daylight Solutions directly for more information on our non-scientific platforms. Scientific systems include:
Tunable Pulsed Pulsed output operation with adjustable pulse width, pulse repetition frequency, and duty cycle. Tunable CW/Pulsed Pulsed or Continuous Wave (CW) operation with narrow line width. Tunable CW-MHF Continuous Wave (CW) operation with Mode Hop Free (MHF) tuning across entire tuning range. Fixed Wavelength Fixed wavelength systems with factory-set center wavelengths for stable and robust operation. Output is factory set to 1 part in 1,000. Pulsed or CW models available.
- What is the difference between CW and pulsed operation?
During pulsed operation, the ECqcL™ device is turned on and off repeatedly. The Pulse Width (PW) and Pulse Repetition Time (PRT) are programmable, and are set by the user via the laser controller.
During Continuous Wave (CW) operation, the laser is on continuously and provides a continuous stream of mid- to long-wave infrared (IR) photons. ECqcL™ lasers operating in CW mode have narrower line widths than pulsed lasers, and are therefore most often used for high-resolution applications.
- What center wavelengths are available?
Daylight Solutions offers products with wavelengths in the 4-12 µm range. Custom wavelengths are available as well. See our Products and Services section for information regarding a broad selection of available “off-the-shelf” wavelengths.
- How broad a wavelength range does each laser tune?
Most Daylight Solutions lasers will tune up to 10% of the center wavelength – although ultra-wide tuning (25% of center wavelength) is available at some wavelengths.
- What does Mode Hop Free (MHF) tuning mean?
Mode hops are caused by internal longitudinal modes competing for available gain within the laser cavity. As the laser tunes across the quantum cascade gain profile, different laser modes become dominant and causes the optical output to “hop” from one mode to another. This “hopping” of the lasers wavelength causes the optical output power to miss certain wavelength bands between the modes. This can cause a spectrometer to miss certain features of an absorption profile.
Daylight Solutions' patented Mode Hop Free (MHF) lasers tune a narrow linewidth continuously across the entire gain band in a way that eliminates mode hopping.
- Does the pulsed laser mode hop?
Yes, but the mode hops are within the line width of the pulsed lasers output.
- How fast do the lasers tune?
The Pulsed and CW/Pulsed lasers can scan across the entire tuning range in about 1 second. The CW-MHF laser is able to scan across the entire tuning range in about 3 seconds. Tuning to discrete wavelengths within the range takes less time, since the wavelength adjustments are smaller. Slower scan rates may be selected based on the demands of a particular application. Custom systems for sensor applications are also available. These systems can tune across the entire band in ~ 1 millisecond.
- What is the output power of each laser?
Daylight Solutions' line of lasers (both Pulsed and CW) typically emits power in the range of several 10's of mW. The entire line of lasers is factory-limited to less than 100mW. Power levels are generally a function of the QCL, and therefore any power requirements should be discussed with your Daylight Solutions sales representative.
Daylight Solutions also offers high-power systems that generate greater than 10 Watts of optical output power.
- What is the resolution and accuracy of the tunable lasers?
The laser controller will display a resolution of 0.1 cm-1 (3.0 GHz), though the laser has a mechanical scanning resolution of 0.001 cm-1 (30 MHz). The accuracy of the laser is +/- 1 cm-1 and the repeatability is 0.02 cm-1.
- What laser is best for my application?
For high-resolution spectroscopy of gas phase molecules with a relatively low molecular weight, the CW-MHF is recommended. For spectroscopy of solid/liquid phase or heavy molecules, the Pulsed and CW-Pulsed laser may be the better choice. Please contact Daylight Solutions to discuss your application so we may offer our best recommendation.
- What additional accessories are needed to complete my laser setup?
- The Pulsed Laser system includes a laser head and controller, and is ready to operate right out of the box.
- The CW operation of the CW/Pulsed laser will require a water chiller.
- The CW-MHF laser will require a water chiller.
- To utilize the PZT for wavelength modulation, a PZT driver and function generator are recommended. Lasers equipped with the current modulation option will also need a function generator to perform Frequency Modulation.
- Can we use our TEC controller in the lab for Daylight Solutions QCLs?
No. Daylight Solutions' laser systems require Daylight Solutions controllers, which among other things include a built-in TEC controller function for optimal thermal regulation of the gain media within the laser head.
- What is the current MTBF estimate for Daylight Solutions QCLs?
Daylight Solutions' patented packaging technology ensures maximum life from QCL gain media. Although Daylight Solutions warrantees their commercial systems for 1 year or 2000 hours of operation, field data indicates systems with over 4,000 hours of operation and still going strong. Under normal operating conditions, Daylight Solutions expects MTBF for QCL devices to reach ~10,000 hours. Visit a QCL supplier's website to see their latest developments.
- What is a linewidth?
The light from any laser is not actually a single wavelength but a narrow band of wavelengths. The width of this band defines the laser linewidth. Various parameters affect the linewidth of a QCL device. The ability of a device to resolve two adjacent absorption lines is a direct function of the devices linewidth. CW lasers tend to have a narrower linewidth than pulsed systems..
- What is the linewidth of the Pulsed laser?
The linewidth of a pulsed laser can be as wide as 1cm-1 (30 GHz), but is more typically ~0.5 cm-1 (15 GHz).
- What is the linewidth of the CW laser?
The average linewidth of the Daylight Solutions CW laser is on the order of < 0.001 cm-1 (30 MHz), with an instantaneous linewidth of <10 MHz, as measured by the ability of the device to resolve 2 adjacent absorption lines that are spaced by 10 MHz.
- What is the difference between Pulse, CW, and CW-MHF operation?
A Pulsed laser emits pulses of coherent light at a given frequency (PRF) and wavelength. A Continuous Wave (CW) laser emits a continuous, uninterrupted stream of coherent light at a given wavelength.
Tuning a CW or pulsed laser in a standard ECqcL™ configuration can cause mode hops at certain frequencies, resulting in gaps in the wavelength of light as the laser “hops” over small bands of frequencies throughout the tuning range. This is not an issue in most applications. However, in some high-resolution applications mode hopping can be an issue. For this reason, Daylight Solutions developed the world's only high-resolution, broadly tunable, mode hop free (MHF) laser. This laser will tune over a large range of frequencies without mode-hoping. This ensures that optical power is emitted at all wavelengths across the entire tuning range of the device.
- What is the range of pulse widths (PW) and pulse repetition frequencies (PRF) that Daylight Solutions offers?
Daylight Solutions offers pulsed ECqcL™ systems with PW from 40 ns to 500 ns, in 20 ns steps. PRF can be adjusted from 100 Hz to 100 kHz in 0.1 kHz steps.
- Can I vary the duty cycle and keep the PW and PRF constant?
No. The duty cycle is a function of the pulse width and the repetition rate. For the duty cycle to vary, either the pulse width or the repetition rate must also vary.
% Duty Cycle is equal to the PW (µsec) divided by the PRT (µsec) times 100.
Example: For a 500nsec PW at 100kHz PRF, %DC= ((0.5µsec)/(10µsec)) X 100 = 5%. This means the laser is emitting power during 5% of the 10µsec cycle.
- Can we use our current supplies in the lab for the Daylight Solutions ECqcLs™?
No. Daylight Solutions laser systems require Daylight Solutions controllers, which among other functions ensure precise control of the current to the ECqcL™. This ensures minimal frequency drift in the laser.
- Can the laser output power be modulated?
Pulsed lasers can be modulated on/off. CW lasers with the current modulation option can be current modulated from 10 Hz to 2.5 MHz, which modulates the power but also translates into 3.8 GHz pp of frequency modulation. The PZT feature of the CW-MHF laser enables independent wavelength modulation of up to 1 cm-1 at up to 100 Hz.
- What communication protocols are available?
You can access the laser controller through the back panel using either the RS-232 serial port or the GPIB port for Labview. The USB interface will be supported in the near future.
- What commands can I execute from the back panel?
All commands available from the front panel can be initiated via computer control through one of the three back panel computer interfaces: GPIB, USB, or RS-232 serial. There are additional commands available through the computer interface alone that allow more control over scanning when incorporating the laser into a larger system such as a spectrometer.
- What is the Beam quality out of a Daylight Solutions ECqcL™?
The output beam is a diffraction-limited Gaussian beam with a beam waist at about 30-50 cm from the exit port. It is a TEM00 mode and has a beam quality factor M2 of typically less than 1.2.
- What is the beam divergence of the Daylight Solutions laser?
All Daylight Solutions ECqcLs™ are guaranteed to have <5 mrad of beam divergence. Most lasers are shipped with a typical value of 3 mrad.
- Are the output beams polarized?
Yes, the Pulsed and CW/Pulsed lasers are linearly polarized in the vertical direction with a greater than 100 to 1 ratio. The CW-MHF laser is rotated 90º so it is horizontally polarized relative to the base of the laser head.
- What is the beam pointing specification over the tuning range?
Since the ECqcL™ is in a Littrow configuration, there is no beam pointing stability problem while scanning across the tuning range. Daylight Solutions guarantees less than 1 mrad of pointing variation for over 100 cm-1 of tuning.
- What materials work best for these wavelengths?
Many materials will work in the 4-12 µm range. ZnSe is one example that also allows a visible tracer beam to be used for optical alignment. Si and Ge also transmit in the 4-12 µm range, but are opaque to visible light.
- What coating should I spec on the optics?
Most optics companies will offer a broad band anti-reflection (BBAR) coating from 3-12 µm. This will allow use of the lenses and windows across the 4-12 µm range of Daylight Solutions lasers.
- When would I use a parabolic mirror instead of a lens?
One advantage of using infrared (IR) mirrors is they do not exhibit chromatic aberration. Chromatic aberration is associated with the fact that different wavelengths have different effective focal lengths for a given lens, and therefore focal point changes for different wavelengths. This can be an issue when using a broadly tunable laser. IR mirrors do not suffer from chromatic aberration and are very useful when focusing a tunable beam onto a detector. Therefore, a parabolic mirror is often a good choice when operating over a wide range of wavelengths.
- What is the difference between a photoconductive (PC) and photovoltaic (PV) detector?
Both are semiconductor detectors, but a PC detector is based on measuring the change in resistance of a piece of semiconductor material when light impinges on it, while a PV detector is based on measuring the photocurrent produced when light impinges on the depletion region of a diode junction. Both types of detectors are sensitive, but PC detectors are limited by 1/f noise at low frequencies, making them unsuitable for DC operation, while PV detectors can be operated in DC mode. PV detectors also have the speed to handle operating at frequencies greater than 1 GHz, while PC detectors peak in the hundreds of MHz.
- What detectors are available for mid infrared work?
There are a wide variety of detector types available - it really depends on the application. For power measurements not requiring a time response faster than 100 msec, pyroelectric and thermopile detectors are a good choice. Both are relatively inexpensive and can be operated at room temperature. For extremely low light levels (< 10 uW), LN2 or TE cooled semiconductor detectors are a must. Semiconductor material and type (PC vs. PV) depend on the application. Common materials include HgCdTe (MCT) over the 1 to 28 µm range, PbSnTe from 2 to 18 um, and InSb from 1 to 5.6 µm. For power levels achievable with QC lasers (1 to 10 mW), room temperature variants of the above detectors are usable. For QC power levels greater than 10 mW, semiconductor detectors are not useful: they either saturate or in some cases can be damaged. It is recommended that pyroelectric or thermopile power meters designed for these power levels be used for power levels greater than 10 mW.
- How do I convert cm-1 (wavenumbers) to µm (microns)?
Wavenumber is a convenient measure of the frequency of coherent electromagnetic radiation and has units of cm-1. Wavenumbers can be pictured as a measure of the number of wavelengths that are present in 1cm of length. If the wavelength of radiation is 1cm, it has one wave in 1cm of length. We know from the relations ship between frequency and wavelength that f = c/λ and therefore 1 cm-1 = 30 GHz. From this we see:
f (cm-1) = 10,000/ λ (µm)
For example if the wavelength is 10 µm than the frequency is 1000 cm-1
- How do I convert GHz to cm-1?
Using the same argument as above the conversion from GHz to cm-1 is given as:
f (GHz) = 30?f (cm-1)
List of Acronyms
- List of Acronyms
%DC Percent Duty Cycle AR Anti-Reflective CW Continuous Wave CW-MHF Continuous Wave – Mode Hop Free DAQ Data Acquisition DFB Distributed Feedback ECicL™ External Cavity Interband Cascade Laser ECqcL™ External Cavity Quantum Cascade Laser FP Fabry-Perot HR High Reflection PRF Pulse Repetition Frequency PRT Pulse Repetition Time PW Pulse Width QCL Quantum Cascade Laser TEC Thermoelectric Cooling
Units of Measure
- Units of Measure
cm-1 Wavenumber µm Micrometer µsec or µs Microsecond msec or ms Millisecond mW Milliwatt µW Microwatt