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TUTORIAL: Laser diodes and pigtailing

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The diode laser is the most compact among all commercially available laser products. The diode's construction begins with a three-leaded header. Mounted on the header resides a monitoring photodiode designed to detect light. Above the photodiode is the laser diode chip where the laser beam is emitted. The laser chip has three layers, two clad layers with an active layer in between. The light beam is emitted from the active layer, also known as the Multiple Quantum Well (MQW).

The glass window on a diode laser not only transmits the light beam, but also forms part of the hermetically sealed package. Once the case is sealed its contents are protected from hazardous elements, unless it is physically destroyed. The glass envelope comes in two different styles, flat window and ball lens.

Applications
Laser diodes are becoming increasingly important in many fields such as the following: Biomedical, photodynamic therapy, entertainment & display, graphic arts, holography, inspection, instrumentation, laser pumping, optical data storage, sensing, LIDAR, pollution monitoring, trace element sensing, spectroscopy, high resolution spectroscopy, Raman spectroscopy, fluorescence spectroscopy, molecular spectroscopy, and telecommunications. Here we list the commonly available diode laser wavelengths and their typical applications:

635   Pointing, holography, replacement of HeNe laser, DVD, CD
650   Pointing, holography, DVD, CD
660   Pointing, holography, DVD, CD
760   Gas sensing
780   CD ROM
808   Solid state laser pumping
850   Communications
980   Erbium-doped fiber pumping
1060 Replacement of low-power Nd:YAG
1310 Communications
1321 Gas sensing
1460 Raman amplifier
1480 Erbium-doped fiber pumping
1540 Gas sensing
1550 Communications, range finding
1578 Gas sensing
1625 Telecommunications testing
1640 Gas sensing
1790 Gas sensing

Ellipticity
The elliptical shaped beam of a laser is caused by having a rectangular single mode waveguide, which requires the NA to be different for each axis. The beam has a different source point for the perpendicular and parallel directions on the active layer of the chip. The angle of the parallel and perpendicular axis of the elliptically shaped beam is known as the beam divergence (8º x 30º). The laser beam, measured at full angle at half the maximum intensity point, diverges much faster along the perpendicular axis.

A number of methods have been commercially used to circularize beams from diode lasers. The most popular include prism pairs, cylindrical lenses, spatial filters, and perpendicular bi-focal cylindrical lenses. However, none of the approaches yields perfect Gaussian beams. By far the best way to circularize a diode laser beam is to couple it into a singlemode fiber. The laser beam emitted from a singlemode fiber is more than just circular in shape; it is also mathematically Gaussian in its intensity profile with a common source point for both directions. A collimated Gaussian beam is ideal for long distance targeting such as Free-Space Optical Communication.

Polarization
The beam from a laser diode has a parallel polarization to its laser junction. Spontaneous emission with random polarization and/or perpendicular polarization to the laser junction also occurs. When the diode is operating near its maximum power, its extinction ratio (ER), which is its parallel polarization component divided by its perpendicular polarization component, is greater than 100:1 (20 dB). When operating near its threshold point, the ratio is considered lower as the spontaneous emission becomes larger.

In general, free-space optics does not affect the SOP (State of Polarization) of a laser beam significantly unless the optical element is either naturally birefringent or contains induced birefringence due to stress. However, once the beam is coupled into an optical fiber, the SOP becomes random unless the fiber is a PM (Polarization Maintaining) fiber. The most popular type of PM fiber is called PANDA, made by Fujikura of Japan, in which the two stress members and the fiber core form a distinctive panda face. Bowtie and Tiger fibers are also popular for a wider range of wavelengths.

All Princetel's pigtailing laser diode modules can be ordered with PM fibers to maintain the SOP of a laser beam. Princetel also offers laser diode module with improved extinction ratio. For example, 28-35 dB extinction ratio can be achieved for 1310 nm and 1550 nm lasers. For those who need to rotate the polarization check out Princetel's rotatable polarized light source.

Back Scattering
All lasers are sensitive to optical back-scattering since it causes the laser frequency to drift and the power to subsequently vary due to the bell curve of the laser's gain profile. DFB (Distributed Feedback) lasers are particularly sensitive. Without isolators DFB lasers will almost certainly fail to operate properly. In general, it is always a good idea to incorporate an isolator into a pigtailed laser package. However, due to material limitation, isolators are currently available only within a couple of wavelength bands. The most commonly available is the telecom band, 1290 nm to 1640 nm. Insertion loss increases to more than 1 dB at wavelength of 1060 nm. The other window is around 780 nm, but the loss goes up to 3-4 dB. Therefore, it is rarely used in packaged devices.

Princetel offers isolator options within the telecom wavelength band. Isolation is typically 40 dB with single-stage or 55 dB with a double-stage isolator.

Fabry-Perot Modulation
Any two optical surfaces can form a Fabry-Perot cavity which would cause spectral modulation. When surface reflection is high enough, modulation amplitude can reach beyond 50%. In the case of trace gas sensing application, extreme care has to be taken in the packaging process to ensure the elimination of Fabry-Perot cavities in the package. Princetel specializes in this type of extreme packaging care. We understand this need of our customers in great detail.

Electro-static Discharge (ESD)
Laser Diodes are very sensitive to ESD and will fail if extreme care is not used in their handling. Common symptoms of laser degradation or failure are decreased laser output power, increased threshold current, or no lasing at all. Proper grounding is critical. Use static-controlled lab equipment and accessories when handling the diode laser at all times.

In the world of laser diodes, there is an entire spectrum of products. Solid state laser diodes have a distinctly different construction making them an intriguing technology. If a solid state laser diode fails, the device will still maintain the characteristics of a diode; only the lasing properties will degrade or cease to exist.

Packaging Styles
Commercially available diode lasers come in a wide variety of package styles. The vast majority of units come in the so called TO can. Most telecom diode lasers are pigtailed in hermetically sealed 14-pin butterfly packages, 14-pin DIP packages, mini DIP packages, or FC/SC receptacle modules. Instrumentation users commonly use collimated laser modules where a lens is used to collimate the laser beam for targeting. Some diode lasers are packaged with a specially designed grating to form a specific beam pattern such as a line, a grid, or multiple dots. If the application requires the mobility of a laser probe, a good quality beam, or mating capability with a fiber, in-line pigtailing or a receptacle module would fit the bill.

Princetel has a full line of standard pigtailed and receptacle packaged laser diodes available. We also package laser diodes with special wavelengths, special fiber types, or special power requirements.

Reliability
A typical diode laser facet is only 0.5 to 3 microns in scale. Sub-micron displacement can cause significant output power degradation. Packaging reliability is a strong measure of a company's technology. The wrong type of bonding agent, improper curing of the agent, mechanical stress due to poor design or process are all possible reasons for concern. Steady optical power degradation of a turnkey laser light source over time is a good indication of poor packaging technology.

At Princetel, we have many years of experience in field applications of pigtailed laser diodes. Our bonding material not only meets NASA's low outgas requirement, but is also capable of withstanding extremely low temperatures (4 degrees Kelvin) without failure.

Single Mode and Multimode Fibers
Optical fibers can generally be classified into two categories: single mode and multimode. When the fiber core is small enough and there can only be one spatial mode, the TEM00 mode, it is single mode, otherwise, it is multimode. PM fibers and common doped fibers, such the erbium doped amplifier fibers, are two special types of single mode fibers because they both support a single mode. Single mode fiber core sizes range from 3 microns to 10 microns depending upon the wavelength. Multimode fibers have core sizes from 50 microns to 1 millimeter.

Coupling efficiency into single mode fiber is around 10-50%. The efficiency goes up to 40-90% when multimode fiber is used due its much larger core size and NA (Numerical Aperture). To determine which fiber is suitable for you, first ask if the beam quality and beam stability is important. If the answer is yes, select single mode fiber and choose an appropriate power level.

Thermo-electrical Cooling (TEC)
Higher temperatures affect a laser diode's threshold current and life expectancy. As the temperature increases, the threshold current of the laser device increases. Therefore a higher driving current is needed to maintain a constant output power level. The life expectancy of a laser diode is expected to decrease by about 50% if the temperature increases 10 ºC. So if the laser will operate under continuous usage with a need for stable power, it would be very beneficial to use temperature control. On the other hand, temperature control may not be necessary for infrequent short runs where power stability is not important.

Consider Princetel's turnkey laser diode light source if temperature control is important. We can provide proper TEC solutions for most laser diode we pigtail.


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