Intermittent CW modulation scheme


One particular interest of quantum cascade lasers is in the narrow intrinsic linewidths (down to <1kHz). The goal is to achieve a low effective linewidth, however, the driving scheme is important.

Three common driving schemes are inter-pulse modulation, intra-pulse modulation and CW modulation. They are described in more details elsewhere but each comes with their limitation:

  • short pulse schemes require either fast current drivers (in the inter-pulse scheme) or fast detectors (in the intra-pulse scheme) to avoid the chirping inherent in pulsed lasers.
  • CW modulation is more demanding on the laser itself and requires large heat dissipation.

We describe here a new scheme, dubbed Intermittent Continuous Wave (ICW) modulation, which allows one to perform spectroscopy with slow detectors and drivers while using lasers in TO-3 cans.  The TO-3 cans are less expensive than the LLH and HHL housing of true CW lasers.

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This scheme was developed in collaboration with the Air Pollution /  Environmental Technology group of EMPA. More information details can be found in the paper published here.

Modulation schemes overview


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The image on the top-left shows a typical CW modulation scheme. The driving current is modulated in a saw-tooth pattern to create a frequency modulation over a 200 us period, which is then followed by a short period below threshold and a repetition. This scheme allows for a slow frequency scanning: if the scanning range is 1 cm-1, then a detector with a 1 us time resolution will yield a spectral resolution of 0.005 cm-1. The small current excursion ensures minimal thermal effects.

Such CW modulation can be used with cooled lasers, for example in a HHL housing. However there are situations where the high footprint and power consumption required for running the laser in a constant-on mode are too high to be sustained.

The ICW scheme, shown on the top-right, diminishes the average dissipation in the laser by dropping the current to zero between pulses and keeping a longer pause between pulses to allow the cooling down of the laser. Doing this limits the overall dissipation and the laser can be used in a TO-3 housing. The thermal excursion is larger which results in a faster transient tuning.

Requirements


ICW lasers must be lasers that have the capability of running in CW mode given enough cooling power. The ICW mode can be applied to any CW laser in a LLH or HHL housing. In addition, the ICW mode can be applied to a similar chip mounted in a TO-3 housing, but in this case a pure CW mode is generally not possible.


Ramps

The tuning rate can be controlled by applying a ramp to the current shape. In this case, the first 40 us of the output is still discarded. Following that, the tuning rate can be increased or decreased by applying a current ramp to increase or decrease the thermal load on the active region of the laser. In this way, the total tuning range within a single pulse can reach up to 2 cm-1.

The following pictures show the typical results. Each lasers will be individually tested.

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Parameter Dependency 

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The overall tuning is almost entirely independent on the temperature of the submount but is dependent on duty cycle. The image on the right shows relative tuning for different temperatures and inter-pulse separation for an identical pulse length.
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The tuning endpoints and the tuning rate are both dependent upon the duty cycle. The image to the left shows absolute tuning with respect to duty cycle. As the pulse-to-pulse separation becomes smaller, the behavior approaches the monochromatic CW result.
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Hardware solutions


Square and sawtooth pulses can be created using programmable CW laser drivers. If you own such a driver you are welcome to use it and we will help you to find the best laser for such an application.

Alpes Lasers has also developed a driver fully dedicated to running lasers in the slow-chirp mode.

An application note is available outlining the performances of the driver in a specific spectroscopic application of nitric oxide detection.

Datasheets and Laser evaluation


Every CW laser mounted on NS mounts can be used in slow-chirp mode in a HHL or LLH housing. The datasheets shown on this website only reflects their performance in pure CW mode. If you inquire about these lasers, please precise the mode in which you intend to use them. 

Since the long current pulse works by heating the laser, it is safe to assume wavelengths available in CW mode will also be available in ICW mode but with the base temperature being colder by about 10°C. The exact temperature shift will be affected by the current used in the laser and the duty cycle. The range available is typically greater than 1.0 cm-1. A specific slow-chirp mode test under your conditions can be performed prior to shipping.

Lasers on NS mounts cannot be mounted in a TO-3 housing. Therefore for a TO-3 laser, please inquire directly by sending us an email.

ICW for practical applications

You can read here of an experiment for High resolution quantum cascade laser spectroscopy of the CH2OO radical performed at the National Taiwan University using the ICW technique.