Picosecond Pulsed Semiconductor Lasers For Time Resolution

Designed and manufactured by Becker & Hickl - bh offers picosecond pulsed diode semiconductor lasers in wavelengths ranging from UV to NIR. All bh picosecond pulsed semiconductor lasers are available with a simple +12V power supply, or from the USB port of a PC or laptop. Other features include high repetition frequencies, short pulse widths, unprecedented timing and power stability, and extremely low electrical noise levels. Complete drive electronics are integrated into the laser module. All bh semiconductor laser modules are directly compatible with bh TCSPC modules. Find out more about the various applications of the products, especially in TCSPC systems, on this page.
What are the applications of ultrashort pulsed lasers?
One of the most important application areas for ultrashort pulsed lasers is the scanning imaging technique of fluorescence lifetime imaging (FLIM). Specifically, the TCSPC-FLIM technique is based on using a pulsed laser beam from a pulsed semiconductor laser to scan a sample at a high repetition rate and then detecting single photons of the fluorescence signal returned from the sample. Each photon is determined by its time in the laser pulse cycle and the position of the laser spot in the scanned area at the time of detection. The recording process creates a photon distribution over these parameters. The result can be viewed as an arrangement of pixels, each containing a complete fluorescence decay curve over a large number of time channels.
Ophthalmic FLIM is one of the application areas for pulsed lasers. The basic requirement for this application is the excitation of the human eye by a picosecond semiconductor laser.
The beam generated by the semiconductor laser is projected directly into the pupil of the patient's eye. The fluorescence returning from the background of the eye (fundus) is detected in two wavelength channels.
This light is captured by the FLIM module, processed and evaluated during imaging. Data obtained in this way provides physicians with the opportunity to recognize early eye disease much faster than currently available methods. Thus, pulsed semiconductor lasers can be of great help in early detection and treatment, ultimately improving the quality of life of patients.
The LHB-104 quad laser box, also known as a "laser-hub", contains up to four BDS-SM lasers. The beams of the individual lasers are combined to form a single free-beam output or a single-mode fiber-coupled output. One box contains the control electronics equivalent to the LSB-C and LSB-C2 laser switch boxes. In addition, the laser box contains wavelength multiplexing electronics, control signal inputs, and TCSPC module synchronization signal outputs.
Becker & Hickl can offer a wide range of lasers for different purposes. Two examples in particular are worth mentioning here:
Single mode laser (BDS-SM)
Multimode laser (BDS-MM)
Due to their visible range, both SM and MM lasers are suitable for special applications. These include the excitation of various fluorophores and other biological samples, as described below.
Two examples of picosecond pulsed semiconductor lasers
bh BDL and BDS lasers are designed for laser scanning microscopy applications. They feature a fast on/off control input that turns the laser off during the scanner's beam flyback and multiplexes multiple lasers of different wavelengths.
BDS-SM Series Lasers
The BDS-SM lasers are small modules measuring only 40 mm x 70 mm x 120 mm. The lasers contain the entire drive electronics. As usual, they are powered by a simple +12 V. The BDS picosecond semiconductor lasers provide both free-beam and single-mode fiber output. Pulse widths are about 50 to 90 ps and the pulse repetition frequency can be switched between 80 MHz, 50 MHz, 20 MHz and CW. Images
All typical semiconductor laser wavelengths from 375 nm to 785 nm are available, other wavelengths are also available on request.The BDS lasers use the same drive principle as the BDL-SMN lasers. As a result, high optical power can be obtained with a good pulse shape, as shown in the figure below. The output power is stabilized by an internal regulation loop and enables fast switching. The laser has a synchronization output to the bh TCSPC module and a trigger input for synchronization with other pulsed lasers.
BDS-MM Series Lasers
The BDS-MM laser is a multimode version of the BDS-SM laser. Depending on the wavelength version, the CW equivalent power at 50 MHz repetition frequency can be as high as 20 to 50 mW. In most cases, the pulse shape is kept free of tails and backpulses up to more than 10 mW. However, some compromises had to be made: due to power consumption limitations, MM lasers do not have continuous modes and light is difficult to focus into the fiber. If possible, BDS-MM lasers should be used with free-beam optics or, if fiber coupling is unavoidable, with multimode fibers with core diameters of 200 μm or larger.
More particularly interesting applications and techniques for picosecond pulsed semiconductor lasers
As shown above, bh's picosecond pulsed semiconductor lasers have many advantages that open up a wide range of applications. Some of these will be depicted here.
Picosecond pulsed semiconductor laser for FLIM with excitation wavelength multiplexing
FLIM can be combined with excitation wavelength multiplexing. An extension of this principle to FLIM is shown below. Excitation at different wavelengths is realized by multiplexing (on/off switching) multiple lasers or by switching the wavelengths of the acousto-optic tunable filter (AOTF) of a supercontinuum laser. The multiplexing signal indicating which laser (or laser wavelength) is active is fed to the routing input of the TCSPC module. The signal indicates the excitation wavelength.
The TCSPC module is running the normal FLIM acquisition process: it builds up a photon distribution over the coordinates of the scanned area, the photon time and the excitation wavelength. The result is a dataset containing images of individual excitation wavelengths. It can also be interpreted as a single image that has multiple decay curves in its pixels for different excitation wavelengths.
Picosecond pulsed semiconductor laser for metabolic imaging
As an example of a very important application of ultrashort pulsed lasers, metabolic imaging is mentioned here. It is based on the simultaneous acquisition of fluorescence lifetime images of NAD(P)H and FAD to minimize the effects of photobleaching, focus drift and possible physiological changes. This can be obtained by laser multiplexing and multiplexing TCSPC. The signals in the two emission wavelength intervals are recorded by two parallel FLIM channels. The core component is the bh DCS-120 confocal scanning FLIM system. Thus, metabolic FLIM using the DCS-120 requires only the use of the correct laser and the selection of the correct setup parameters.
The adjacent figure illustrates the performance of the system using human bladder cells. tm images, a1 images and FLIRR images can distinguish between normal and tumor cells. The data obtained from metabolic imaging are invaluable for therapy.
Synchronized FLIM/PLIM using ultrashort pulsed lasers
In addition, picosecond pulsed semiconductor lasers are a key component of synchronized FLIM/PLIM.
In contrast to other techniques, not one but several laser pulses are used per phosphorescence excitation cycle.
The excitation laser of a FLIM system is modulated with cycles in the microsecond or millisecond range.
The system generates FLIM images based on the photon time in the laser pulse cycle and PLIM images from the time in the modulation cycle. The adjacent figure illustrates the principle.
Picosecond pulsed semiconductor laser and spatial multiplexing
A combination of wavelength multiplexing and spatial multiplexing is used for diffuse optical tomography (DOT). The principle is illustrated in the figure below. Several picosecond semiconductor laser beams are combined into a single fiber and multiplexed. The fiber with the combined laser is connected to the input of a fiber optic switch or divided into sections connected to multiple fiber optic switches. The fiber switch multiplexes the laser light (which itself consists of several multiplexed laser wavelengths) continuously into a large number of fibers that deliver the light to different sample locations.
Diffusely transmitted light is recorded by a large number of detectors at other locations in the sample. Detector signals are recorded by parallel TCSPC modules with "channel" inputs for recording signals from different source locations and laser wavelengths into different waveform memory blocks. In order to increase the number of detector positions, the setup can be extended by a router.