Volume-Holographic-Grating- (VHG) Stabilized SF Lasers, TO Cans

Webpage Features
	Clicking this icon opens a window that contains specifications and mechanical drawings.
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Choose Item	Clicking the words "Choose Item" opens a drop-down list containing all of the in-stock lasers around the desired center wavelength. The red icon next to the serial number then allows you to download L-I-V and spectral measurements for that serial-numbered device.

Features

• 785 nm, 808 nm, 852 nm, or 976 nm Center Wavelength
• Wavelength-Stabilized Output
• Ø9 mm TO Can Package

Applications

• Raman Spectroscopy
• Microscopy

Thorlabs' Volume-Holographic-Grating- (VHG) Stabilized Lasers are laser diodes that use feedback from a volume holographic grating to provide narrow-linewidth, single-frequency operation. This allows the laser to achieve 10 MHz typical linewidths for Item #s LD785-SEV300, LD808-SEV500, and LD852-SEV600, and excellent side mode suppression ratios (typically 40 dB or higher) for all of the VHG stabilized laser diodes on this page. Typical performance graphs can be viewed by clicking on the blue icons () in the tables below. More information on the stabilized temperature range can be found on the individualized data sheet for each laser diode.

These VHG-stabilized lasers are housed in compact Ø9 mm TO can packages with E pin codes. As they do not contain internal isolators, we recommend using an NIR Optical Isolator when the output is collimated. Single-frequency performance is only guaranteed with >35 dB isolation from back reflections. We also offer VHG lasers in butterfly packages and a 785 nm laser in an SM fiber pigtail package with an FC/APC connector (Item # LP785-SAV50).

While a center wavelength is listed for each laser diode below, this is only a typical number. The center wavelength of a particular unit varies from production run to production run, so the diode you receive may not operate at the typical center wavelength. After clicking "Choose Item" below, a list will appear that contains the center wavelength, output power, and operating current of each in-stock unit. Clicking on the red Docs Icon next to the serial number provides access to a PDF with serial-number-specific L-I-V and spectral characteristics.

Laser diodes are sensitive to electrostatic shock. Please take the proper precautions when handling the device, such as using an ESD wrist strap.

Thorlabs also offers pigtailed volume-holographic-grating-stabilized and distributed feedback (DFB) single-frequency lasers and external cavity and distributed bragg reflector (DBR) butterfly-packaged single-frequency lasers. The DFB lasers provide similar linewidths to the VHG-stabilized lasers, but provide mode-hop free tuning of several nanometers while maintaining single-frequency operation. Our DBR single-frequency lasers offer a similar linewidth to the VHG-stabilized lasers, but have a temperature- and current-tunable center wavelength. A description of the differences between each type of laser is provided on the SFL Guide tab.

Video Insight: Setting Up a TO Can Laser Diode

Installing a TO can laser diode in a mount and setting it up to run under temperature and current control presents many opportunities to make a mistake that could damage or destroy the laser. This step-by-step guide includes tips for keeping humans and laser diodes safe from harm.

ECL, DFB, VHG-Stabilized, and DBR 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. 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 these four types of 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 (several hundred kHz). 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 with TO can, pigtailed TO can, and butterfly packaging, and MIR with two-tab C-mount, D-mount, and HHL packaging) 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 from several hundred kHz to 10 MHz. 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, and DBR 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 also provides the narrowest linewidth (several hundred kHz) of the three choices. 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 of the four. 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 often has low output power due to inherent properties of the continuous grating feedback structure, but higher powers can be achieved with different packaging styles.

The VHG laser provides the most 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.