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  • 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.

  • Beam Quality

    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.

  • Infrared Optics

    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.

  • Infrared Detectors

    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.

  • Other FAQs

    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
    %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
    cm-1 Wavenumber
    µm Micrometer
    µsec or µs Microsecond
    msec or ms Millisecond
    mW Milliwatt
    µW Microwatt

     

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