Single-Frequency, Ultra-Low-Noise Laser, Extended Butterfly Package

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Features

• 1550 nm Typical Center Wavelength
• Less than -165 dBc/Hz Relative Intensity Noise (RIN)
• 14-Pin Hermetically Sealed Extended Butterfly Package
• Two Integrated Thermoelectric Coolers (TEC) and Thermistors
• PM Fiber Pigtail with 2.0 mm Narrow Key FC/APC Connector
• Requires User-Supplied Laser Mount and Low-Noise Current Source

Thorlabs' Ultra-Low-Noise (ULN) Hybrid Lasers provide signals with relative intensity noise below -165 dBc/Hz and instantaneous (Lorentzian) linewidths of <100 Hz. The single mode output can reach optical powers of 140 mW with a side mode suppression ratio (SMSR) of 70 dB. These specifications are achieved through a patented design consisting of a single angled facet (SAF) gain chip coupled to a fiber Bragg grating (FBG) enclosed in a hermetically sealed case.¹ Two integrated thermoelectric coolers (TECs) and thermistor pairs act in a closed loop to control the temperature of the gain chip and FBG independently. In addition, each laser features an auxiliary thermistor that monitors the case temperature and a built-in monitor photodiode.

The lasing spectrum of these sources changes between single longitudinal mode and multimode depending on the drive current. As the system experiences effects of hysteresis, the width of these ranges will also change depending on whether the drive current is being increased or decreased. Each laser's center wavelength is tested and reported on a serialized data sheet; this wavelength may be anywhere in the ITU C band (1530 - 1565 nm), but most of the devices below will operate at or near 1550 nm. Users should reference the laser's individual data sheet to see where these regions exist for their device. This is discussed further in the Operation tab above.

Specifications for each item can be found in the tables below and by clicking on the blue icons (). These specifications are typical values and the performance will vary from device to device. Each ultra-low-noise laser diode is serialized and shipped with individual test data. Customers can click "Choose Item" below to view a variety of available center wavelengths. The reported specifications are achievable by using a low-noise current supply. Customers that require a specific wavelength may contact tech support to place a special order.

Thorlabs does not offer a current supply with sufficiently low noise to drive this laser at the reported specifications; our testing was performed with a Vecsent Photonics D2-105 Laser Controller. The chosen current supply should be given time to stabilize before the output of the laser is measured. In addition, an optical isolator should be placed at the output of the laser. We recommend the IOT-G-1550A dual-stage isolator, which can achieve a peak isolation of 55 dB; alternatively, two fiber-coupled isolators can be used inline to achieve a similar isolation. 

Current-Tuning vs Temperature-Tuning Configurations
Our ultra-low-noise lasers are available in either a current- or temperature-tuning configuration. The current-tuning configuration is designed to maximize the range of drive currents where the laser operates in a single mode. The temperature-tuning configuration allows for independent adjustment of the FBG temperature without impacting the width of the mode-hope-free range. Users requiring the maximum possible mode-hop-free tuning and/or current range should select the current-tuning configuration, while users wishing to thermally tune the output wavelength without impacting the output power should select the temperature-tuning configuration. See the subgroups below for more information on the differences between these configurations. For more information on how these devices operate, see the Operation tab above.

Mounting and Care
Each laser is housed in an extended 14-pin butterfly-style package. This extended package is not compatible with standard 14-pin laser diode mounts and must be affixed to a purpose-built heat sink. The laser's case has four mounting slots for use with 2-56 (M2) screws. Note that the ultra-low-noise hybrid lasers are extremely sensitive to both seismic and acoustic vibrations, so a mounting fixture that dampens vibration is suggested. Thorlabs offers packs of fifty 2-56 screws (SH2S019) as well as a 2-56 tap (TAP256) to aid in installation. Mounting torque should not exceed 0.14 N•m (20 oz-in); this can be applied and checked using a torque driver.

We recommend cleaning the fiber connector before each use in case any dust or other contaminants have been deposited on the surface. The laser intensity at the center of the fiber tip can be very high and may burn the tip of the fiber if contaminants are present. While the connector is cleaned and capped before shipping, we cannot guarantee that it will remain free of contamination after it is removed from the package. We also recommend that the laser is turned off when connecting or disconnecting the device from other fibers.

Single Longitudinal Mode Operation

Thorlabs' ULN hybrid lasers exhibit fluctuations between single and multimode operation. These regions of stable output change based on a number of factors.¹ Here, we will describe how the drive current changes the single mode operating regions. Note: the following data is typical; users should reference the laser's serialized data sheet to see the drive currents that provide stable single mode operation for their particular device.

Longitudinal Mode Transitions
The fluctuations between single mode and multimode operation are due, in part, to the supported lasing modes within the cavity created between the high-reflectance back facet of the gain chip and fiber Bragg grating. The graph below shows the single mode powers for a representative ULN15PT temperature-tuning laser. Drive currents between the single mode ranges will result in multimode outputs. For that reason, the resulting powers were removed from the graph. As an example, when driven at approximately 220 mA, this unit will generate multimode output, while a 300 mA drive current will produce a single mode output. Current-tuning configurations of these lasers are designed to minimize these multimode regions.

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Each single mode region is shown, while multimode regions have been removed.

Hysteresis
The single mode regions of the ULN hybrid lasers are not a static width; they change depending on whether the current is increased or decreased after single mode operation is reached. The graph below is of a representative ULN15PC current-tuning laser's power output when increasing and decreasing the current. Consider the laser's performance when driven with currents between 700 and 725 mA. If the current is increased from 700 mA, the output will not become single mode until a current of approximately 715 mA (red curves on the graph below); however, after entering this region of single mode operation, the drive current can be lowered to almost 700 mA with the laser cavity maintaining single mode operation (blue curves). When driving a ULN hybrid laser, it is important to keep in mind the direction of the current change to maintain stable operation.

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The range of currents which produce single mode outputs is wider for decreasing currents compared to increasing currents.

Longitudinal Mode Shifts with T_FBG
The grating period of the ULN laser's internal FBG changes based on its temperature. When using a temperature-tuning ULN laser, independently setting this temperature helps define the cavity's supported lasing modes and tune to a desired output wavelength. However, because the FBG temperature influences the supported lasing modes, the single mode regions shift. The graph below shows how the single mode operating regions change when adjusting the T_FBG in a typical ULN15PT temperature-tuning laser. For a large enough temperature change, the laser will eventually move from single mode to multimode operation. If the laser is driven at a current at the edge of a region of single mode operation, this change can occur over just a few degrees Celsius. For this reason, we recommend monitoring the laser's output with an optical spectrum analyzer while temperature tuning to ensure that the output remains single mode.

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Single mode regions will shift as T_FBG changes, which may cause the laser to fall out of single mode operation at a fixed drive current.

Wavelength Shift
In addition affecting the laser's modes, changes in drive current will also shift the output wavelength. A representative graph of a ULN15PC laser's wavelength can be seen below. Each group of colored points indicates a mode-hop-free, single mode operating region. The same wavelength/current dependence can be seen in our temperature-tuning models; however these models are designed to provide wavelength tuning at a constant power by changing the fiber Bragg grating temperature. Note: the stepping structure of the center wavelengths is due to the resolution of the optical spectrum analyzer used to take the measurement; the change in wavelength is smooth across each region of single mode operation.

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Within single mode operating regions, the center wavelength will shift with current. Each trendline indicates a region of single mode performance. The stepping structure of the measured data is due to the resolution of the optical spectrum analyzer used to take the measurement; the change in wavelength is smooth when in single mode operation.

Extended Butterfly Package Pin Diagram

ECL, DFB, VHG-Stabilized, DBR, and Hybrid Single-Frequency Lasers

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Figure 1: ECL Lasers have a Grating Outside of the Gain Chip

A wide variety of applications require tunable single-frequency operation of a laser system. In the world of diode lasers, there are currently four main configurations to obtain a single-frequency output: external cavity laser (ECL), distributed feedback (DFB), volume holographic grating (VHG), and distributed Bragg reflector (DBR). All four are capable of single-frequency output through the utilization of grating feedback. In addition, an ECL can be combined with a fiber Bragg grating (FBG) to create a hybrid design. However, each type of laser uses a different grating feedback configuration, which influences performance characteristics such as output power, tuning range, and side mode suppression ratio (SMSR). We discuss below some of the main differences between single-frequency diode lasers.

External Cavity Laser
The External Cavity Laser (ECL) is a versatile configuration that is compatible with most standard free space diode lasers. This means that the ECL can be used at a variety of wavelengths, dependent upon the internal laser diode gain element. A lens collimates the output of the diode, which is then incident upon a grating (see Figure 1). The grating provides optical feedback and is used to select the stabilized output wavelength. With proper optical design, the external cavity allows only a single longitudinal mode to lase, providing single-frequency laser output with high side mode suppression ratio (SMSR > 45 dB).

One of the main advantages of the ECL is that the relatively long cavity provides extremely narrow linewidths (<1 MHz). Additionally, since it can incorporate a variety of laser diodes, it remains one of the few configurations that can provide narrow linewidth emission at blue or red wavelengths. The ECL can have a large tuning range (>100 nm) but is often prone to mode hops, which are very dependent on the ECL's mechanical design as well as the quality of the antireflection (AR) coating on the laser diode.

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Figure 2: DFB Lasers Have a Bragg Reflector Along the Length of the Active Gain Medium

Distributed Feedback Laser
The Distributed Feedback (DFB) Laser (available in NIR and MIR) incorporates the grating within the laser diode structure itself (see Figure 2). This corrugated periodic structure coupled closely to the active region acts as a Bragg reflector, selecting a single longitudinal mode as the lasing mode. If the active region has enough gain at frequencies near the Bragg frequency, an end reflector is unnecessary, relying instead upon the Bragg reflector for all optical feedback and mode selection. Due to this “built-in” selection, a DFB can achieve single-frequency operation over broad temperature and current ranges. To aid in mode selection and improve manufacturing yield, DFB lasers often utilize a phase shift section within the diode structure as well.

The lasing wavelength for a DFB is approximately equal to the Bragg wavelength:

where λ is the wavelength, n_eff is the effective refractive index, and Λ is the grating period. By changing the effective index, the lasing wavelength can be tuned. This is accomplished through temperature and current tuning of the DFB.

The DFB has a relatively narrow tuning range: about 2 nm at 850 nm, about 4 nm at 1550 nm, or at least 1 cm^-1 in the mid-IR (4.00 - 11.00 µm). However, over this tuning range, the DFB can achieve single-frequency operation, which means that this is a continuous tuning range without mode hops. Because of this feature, DFBs have become a popular and majority choice for real-world applications such as telecom and sensors. Since the cavity length of a DFB is rather short, the linewidths are typically in the 1 MHz to 10 MHz range. Additionally, the close coupling between the grating structure and the active region results in lower maximum output power compared to ECL and DBR lasers.

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Figure 3: VHG Lasers have a Volume Holographic Grating Outside of the Active Gain Medium

Volume-Holographic-Grating-Stabilized Laser
A Volume-Holographic-Grating-(VHG)-Stabilized Laser also uses a Bragg reflector, but in this case a transmission grating is placed in front of the laser diode output (see Figure 3). Since the grating is not part of the laser diode structure, it can be thermally decoupled from the laser diode, improving the wavelength stability of the device. The grating typically consists of a piece of photorefractive material (typically glass) which has a periodic variation in the index of refraction. Only the wavelength of light that satisfies the Bragg condition for the grating is reflected back into the laser cavity, which results in a laser with extremely wavelength-stable emission. A VHG-Stabilized laser can produce output with a similar linewidth to a DFB laser at higher powers that is wavelength-locked over a wide range of currents and temperatures.

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Figure 4: DBR Lasers have a Bragg Reflector Outside of the Active Gain Medium

Distributed Bragg Reflector Laser
Similar to DFBs, Distributed Bragg Reflector (DBR) Lasers incorporate an internal grating structure. However, whereas DFB lasers incorporate the grating structure continuously along the active region (gain region), DBR lasers place the grating structure(s) outside this region (see Figure 4). In general a DBR can incorporate various regions not typically found in a DFB that yield greater control and tuning range. For instance, a multiple-electrode DBR laser can include a phase-controlled region that allows the user to independently tune the phase apart from the grating period and laser diode current. When utilized together, the DBR can provide single-frequency operation over a broad tuning range. For example, high end sample-grating DBR lasers can have a tuning range as large as 30 - 40 nm. Unlike the DFB, the output is not mode hop free; hence, careful control of all inputs and temperature must be maintained.

In contrast to the complicated control structure for the multiple-electrode DBR, a simplified version of the DBR is engineered with just one electrode. This single-electrode DBR eliminates the complications of grating and phase control at the cost of tuning range. For this architecture type, the tuning range is similar to a DFB laser but will mode hop as a function of the applied current and temperature. Despite the disadvantage of mode hops, the single-electrode DBR does provide some advantages over its DFB cousin, namely higher output power because the grating is not continuous along the length of the device. Both DBR and DFB lasers have similar laser linewidths. Currently, Thorlabs offers only single-electrode DBR lasers.

Ultra-Low-Noise Hybrid Laser
Thorlabs Ultra-Low-Noise (ULN) Hybrid Lasers each consist of a single angled facet (SAF) gain chip coupled to an exceptionally long fiber Bragg grating (FBG). They are designed to create a laser cavity, similar to an ECL, through the length of fiber. This cavity provides the ULN hybrid laser with a very narrow line width on the order of 100 Hz and low relative intensity noise of -165 dBc/Hz (typical). The FBG reflects a portion of the light emitted from the gain medium while remaining thermally isolated from it. The grating period can be changed by introducing thermal stress to the fiber, allowing users to temperature tune the laser output while being able to independently stabilize the gain medium's temperature. Because the laser's configuration provides excellent low-noise performance, it is likely the laser will not be the limiting factor at low-noise levels. It is critical to monitor the laser's environment to limit external noise contributions like acoustic and seismic vibrations, as well as driving the laser with a low-noise current source.

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Figure 5: Thorlabs Hybrid Lasers have a Fiber Bragg Grating Coupled to the Active Gain Medium

Conclusion
ECL, DFB, VHG, DBR, and hybrid laser diodes provide single-frequency operation over their designed tuning range. The ECL can be designed for a larger selection of wavelengths than either the DFB or DBR. While prone to mode hops, it provides narrow linewidths (<1 MHz). In appropriately designed instruments, ECLs can also provide extremely broad tuning ranges (>100 nm).

The DFB laser is the most stable single-frequency, tunable laser configuration. It can provide mode-hop-free performance over its entire tuning range (<5 nm), making it one of the most popular forms of single-frequency laser for much of industry. It has the lowest output power due to inherent properties of the continuous grating feedback structure.

The VHG laser provides stable wavelength performance over a range of temperatures and currents and can provide higher powers than are typical in DFB lasers. This stability makes it excellent for use in OEM applications.

The single-electrode DBR laser provides similar linewidth and tuning range as the DFB (<5 nm). However, the single-electrode DBR will have periodic mode hops in its tuning curve.

Hybrid lasers can be used to achieve extremely low-noise signals. In order to take advantage of this characteristic, the laser must be isolated from unwanted noise sources, such as acoustic and seismic vibrations and drive current noise.