1. Introduction
Since 1992, Superlum has been manufacturing different types of broadband light sources. Our main products are Superluminescent Diodes (SLD) and SLD-based light sources. The main advantage of SLDs over other types of light sources is a combination of high brightness (comparable to that of laser diodes) and a very wide and flat optical spectrum (with a width comparable to that of light-emitting diodes). SLDs were initially developed for use as light sources in fiber-optic gyroscopes and fiber-optic electric current sensors. Now, they are widely used in other applications such as medicine, spectroscopy, metrology, etc.
In 2007, we started offering another type of light sources—Swept Wavelength Tunable Laser Sources. These devices, which are external cavity semiconductor lasers, feature the ability of sweeping the lasing wavelength over a wide spectral range, that makes it possible to obtain a wide time-averaged spectrum (preserving a narrow instantaneous linewidth). Our tunable lasers have a very wide tuning range and a very short wavelength tuning time for any wavelength within the full tuning range. They can sweep the wavelength repeatedly with very high rate, either over the full tuning range or over a limited operating range. The key element of such a laser is a broadband gain module that can contain either a gain chip or a semiconductor optical amplifier (SOA) chip. These gain chip modules and SOA modules are available as separate items as well, so you can use them for your particular purposes, for example, for developing your own tunable laser.
This document has been prepared to help you greatly simplify the process of selecting the product. Based on over 16 years of working with our customers, this document outlines the major criteria you should consider and contains information needed to successfully select the product, saving your time and efforts. If you are new to SLD, you should probably begin with reading our "Short overview of device operation principles and performance parameters ".
2. SLDs and SLD-based light sources.
2.1. Selection criteria to consider.
Wavelength. Select the most appropriate SLD wavelength/spectral band. Superlum offers SLDs (with different output power and spectrum width) at 650, 670 – 690, 770 – 860, 910 – 980, 1020 – 1200, 1270 – 1330, 1400 – 1630 nm spectral bands. Select the desired wavelength of SLD emission. See "SLD Modules" page, for all wavelengths available.
There are lot-to-lot variations of SLD wavelength within each spectral range. If exact SLD wavelength is critical parameter for your application, please contact our support team to discuss your requirements before placing a purchase order. Otherwise, you can receive SLD module at any peak wavelength within the limits indicated in our datasheets.
Output power. Determine what optical power you need for your application. Do not choose SLD with the output power far above your practical needs. Selecting SLD with higher optical power will increase the price of the SLD module and require more severe safety precautions for using the SLD. High-power SLDs are extremely sensitive to optical feedback, so, if such an SLD is required for your application, you should consider using an appropriate optical isolator. For more information about safety precautions, please refer to the following application note:
"SLD sensitivity to optical feedback".
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Spectral width. We offer various lines of SLD modules with different widths of the emission spectrum. Some SLDs provide exceptionally wide spectrum. However, we do not recommend to select SLDs with a broader spectrum than you really need. Such SLDs may require additional effort to integrate them into your optical system. Their spectrum may have complicated shape and, in addition, may strongly depend on output power. Please note that the SLD spectrum width indicated in our specifications is guaranteed only at the rated output power, and is not guaranteed at a different output power (if it is far from the rated value). Please also make sure that optical bandwidths of other elements of your optical system, like filters, couplers, splitters, isolators, and so on, are large enough to accommodate the broad SLD spectrum. Otherwise, you may just waste power and spectrum of a broadband SLD and don't get the performance you paid for.
Please refer to the following documents for more information:
If the spectrum of a single SLD is not wide enough to fit your application needs, consider our "Broadlighter" family of light sources.
Superlum Broadlighters Product Family:
Spectral ripple. Please pay special attention to the spectral ripple of SLD. The main cause of SLD spectral ripple is the residual reflections from the cleaved end facets of the semiconductor chip. We put in a lot of effort to minimize these reflections but parasitic spectral modulation may still present at the top of SLD spectrum, especially for high-power SLDs. The spectral ripple value in our specifications is calculated as spectral modulation depth ([Imax-Imin]/[Imax+Imin]) averaged over 10 periods around its maximum value at the top of the spectrum, where Imax and Imin are the maximum and minimum peak values of the modulated spectrum, respectively. It is not guaranteed that a particular module we ship will have a spectral ripple equal to the typical value quoted in the specifications. It is only guaranteed that spectral ripple will be less than the specified maximum value. Please discuss the SLD ripple issue with us if its exact value is important for your application. In some cases we can ship very high-power SLDs with the spectral ripple being so low that it is virtually unmeasurable by modern diffraction grating optical spectrum analyzers (OSAs).
Secondary coherence sub-peaks. Superlum was the first company that added a new important parameter-secondary coherence sub-peaks in SLD coherence function-in the device ratings. The cause of these sub-peaks is residual spectral ripple described above. Their maximum value is extremely important for interferometer systems, being one of the main parameters limiting system performance. It is important that Superlum measures secondary coherence sub-peaks directly, by Michelson interferometer. This method eliminates the underestimation of secondary coherence effects inherent in the method of calculating the coherence function from the measured spectrum (inaccuracy of the calculating method is caused by limited spectral resolution of diffraction grating OSAs used for measuring). It is important that secondary coherence sub-peaks may be different in SLDs with the same averaged ripple because they depend not only on averaged value of ripple but also on the fine structure of ripples across the entire SLD spectrum. If your application is optical interferometry, or similar, we recommend to focus your attention on secondary coherence effects rather than upon spectral ripples.
Far-field pattern. Determine whether you need an SM-fiber coupled or a free-space SLD module. If you need a free-space module, please take into account the specifics of its far field. The far field of the most of our SLDs is crescent-shaped. It is caused by tilting the active waveguide with respect to output facet in most of our SLDs. If you are concerned that this shape of the far field may present a problem for your application, please let us know before placing an order.
Take a look at Figure 5 in "Short Overview of Device Operation Principles and Performance Parameters" for an example of the far field of a free-space SLD module at 680 nm.
Note that SM-fiber-coupled SLDs may be suitable for some free-space applications because of easy light delivery and easy reshaping of light beam after the SM fiber end. Light from an SM fiber is a diffraction-limited, low-N.A. (0.1-0.14 depending on fiber characteristics and on spectral band) circular beam.
Polarization. The emission of most of our SLDs is partially or substantially polarized (polarization extinction ratio exceeds 100:1 in some specific models). But SM-fibers do not maintain polarization. Nevertheless, it is possible to get a definite polarization at the fiber output by using polarization-maintaining (PM) fiber pigtailed SLDs. Our technology of PM fiber alignment and fixing in a module provides in-situ control of the angular position of the PM fiber with respect to the SLD emitter. For the standard configuration of our PM-fiber coupled modules, the main polarization direction is oriented along the slow axis of the PM-fiber. We can also ship pseudo-depolarized versions of our SLD modules (light is launched into the fiber with its polarization oriented at 45 degrees to the birefringent axes). However, please keep in mind that in this case the beam coming out of the fiber is not completely unpolarized. It consists of varying amounts of circularly and elliptically polarized light at different wavelengths and may be considered as depolarized only being averaged over the entire spectrum, without any spectral filtering. Pseudo-depolarization may cause problems for some polarization sensitive instruments (for example, for a polarization sensitive interferometer, ghost subpeaks may appear in the coherence function).
Feel free to contact us if you are unsure about the best choice of SLDs. We need just a few details about your application to make more specific recommendations about what types of SLD or SLD-based light sources to choose.
2.2. Drive current and temperature requirements: the problem and our solution.
Once you have selected an SLD module, it's time to decide on the appropriate driver for your SLD.
Most of our SLD modules need active cooling and require the use of a temperature controller (these SLD modules have built-in thermistors and thermoelectric coolers which must be connected to a temperature controller). The output power of an SLD depends on the temperature of the SLD chip: the power decreases as the temperature increases. The temperature controller maintains a constant SLD chip temperature, thus keeping the output power at a constant level.
Besides, SLDs, like laser diodes, are very sensitive to instabilities in the driving current, such as surges, transients, etc. These instabilities can cause irreversible damage to the SLD chip. In most cases, the SLD cannot be repaired from that kind of damage. To prevent damage, a special current controller, just as for laser diode applications, is required. If you are unsure what current controller to choose, we recommend you to use our PILOT driver that has been specially developed for applications of this kind. It combines the functionality of a current and a temperature controller in a single unit and features very high reliability and safety for driving an SLD. Please note that we consider any failure of an SLD caused by improper driving to be covered by the warranty only if the SLD is used with our PILOT driver.
Superlum manufactures several models of drivers. We recommend starting from our 110/220 V AC-powered drivers even if you consider changing to OEM PCB drivers in the future. AC-powered PILOTs will allow you to start testing an SLD in your equipment immediately after you receive it with minimized chances for SLD instability or damage.
We also recommend you to consider our high-power, optically isolated S-series light sources. We offer both "S-series Broadlighter" AC-powered benchtop light sources and OEM-oriented DC-powered products named S-series BLM modules. We recommend you to choose a turnkey S-series Broadlighter if you need a light source ready to go right out of the box, for example for optical component characterization system. You can also start from a BLM light source module if you are planning to integrate this light source into your equipment. In the latter case, you will need a high-quality (with very low ripple and noise) and highly reliable regulated DC power supply and a relatively simple external control circuit for remote control of light source status. If our standard products do not fit your needs, feel free to contact us with your detailed requirements. Just let us know basic performance parameters and the configuration required. We will use all our knowledge and experience to find the most fast and effective way to develop the required light source for you.
Superlum S-series light sources:
2.3. Multiple SLD light sources: beam combining for ultra-broadband applications.
If you are looking for a compact light source with an extremely broad spectrum and a short, say just a few microns, coherence length, please consider our D-, T- and Q-series "Broadlighters". A "Broadlighter" light source combines light from multiple SLD modules with slightly shifted central wavelengths. D-, T- and Q-series light sources contain two, three, and four SLD modules, respectively. We specially recommend our D-855 Broadlighter (> 10 mW ex fiber, 100-nm spectrum width by just two SLDs); D-890 with 140-nm-wide spectrum, also by just two SLDs; T-, Q-870 with 170-200-nm-wide spectrum; and Q-940 which covers 300 nm by four SLDs. Let us point out again that, to avoid narrowing the spectrum and wasting optical power, all optical components in your system should have sufficient bandwidth to handle the broad spectrum of Broadlighters.
Superlum Broadlighter product lines:
3. Swept wavelength tunable laser sources and gain/SOA modules.
The Broadsweeper is a tunable external cavity semiconductor laser. Acousto-optic tunable filter (AOTF) is used as a spectrally selective intracavity element for tuning and sweeping the laser wavelength. High-precision control systems and the absence of moving parts in the laser cavity ensure rapid wavelength tuning with high accuracy and repeatability. The use of a temperature-stabilized AOTF ensures excellent stability of output emission parameters (set values of lasing wavelength, sweep range limits, etc.) during the lifetime of a laser.
More information about Superlum Broadsweeper product.
Please contact us if you need a special law of variation of wavelength in time (for example, if you need the Broadsweeper to rapidly hop over, say, 10 wavelengths in a given order). A law of wavelength variation can be preset at the factory by modification of the Broadsweeper firmware program.
Feel free to request a laser of this kind at a spectral band other than that of a standard product. We can develop such a tunable laser at any spectral band in which our SLDs are available, because any SLD chip is actually a highly effective optical amplifier.
If you plan to build your own external cavity laser, consider our gain chip modules and SOA modules. They have been specially developed for use as an active element for external cavity lasers. Using gain chip modules is the easiest way to build a laser of this kind. However, due to high-reflection coating on the back facet of a gain chip, only the simplest configuration (extended-cavity laser) can be built. Our SOA modules don't have this limitation and allow you to build any configuration of external cavity laser, including a ring-cavity configuration.
You may as well use our SOAs in your systems for amplification of weak optical signals.
Please note that, for the most of our SOAs and gain chip modules, the gain is very sensitive to the polarization of light to be amplified. So, we recommend considering our PM-coupled SOAs and gain modules. For these modules, the main polarization is oriented along the slow axis of a PM-fiber. This is achieved by proper rotational alignment of the fiber with respect to SOA/gain chip during the fiber alignment and fixing in a module.
Please note that gain chips and SOAs require safety precautions similar to those for SLDs. In particular, always use the same if not more precautions when driving them. Some SOAs provide fiber-to-fiber gain exceeding 30 dB. Lack of permanent control of their power may result in immediate failure due to catastrophic optical damage. This may happen even in case of a minor, at first sight, reconfiguration of an external cavity or in case of small variation of input signal (if you use a SOA module to amplify a weak optical signal).
Please contact us if you have any questions or need additional information about our products.
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