Low-Noise, Narrow-Linewidth Laser Systems, 1550 nm

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The ULN15TK laser can be mounted to an optical table using four 1/4"-20 cap screws. The ULN15TK is pictured with a P3-1550PM-FC-5 PM fiber patch cable and three CA2912 SMA cables, which are connected to the optical output and analog modulation ports, respectively.

Reza Salem
BU Leader, Fiber Lasers

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Thorlabs' single-frequency, turnkey, low-noise laser systems are ready-to-use laser systems that integrate our ULN15PC or DFB1550P diodes with a low-noise driver and temperature stabilization inside of a benchtop housing. To minimize the laser linewidth and relative intensity noise (RIN), these lasers include drive electronics that are designed to minimize the current noise. An isolator is integrated at the output of the lasers to minimize the impact of any back-reflected light on the lasers.

The ULN15TK is an ultra-low-noise (ULN) laser system based on a hybrid external-cavity semiconductor laser employing a fiber Bragg grating (FBG) for feedback to the laser cavity. This patented design enables exceptionally narrow Lorentzian linewidths of 100 Hz and record low RIN below -160 dBc/Hz. When factory-set for optimized performance, this laser offers typical optical output powers of 120 mW with an SMSR of 70 dB. As a result of this unique design, the mode-hop-free single-frequency regions are not continuous as the laser current is adjusted, typically limited to 20 mA to 50 mA wide current regions. The temperature of the laser case is stabilized through a third temperature stabilization circuit to provide long-term power and wavelength stability when the system is used in standard laboratory environments.

The DFB15TK is based on a distributed feedback (DFB) cavity on the laser chip, providing single-frequency emission with typical relative intensity noise (RIN) of -150 dBc/Hz and typical linewidths of 50 kHz. Factory-set for optimized performance, this laser offers a minimum output power of 100 mW with a side-mode suppression ratio (SMSR) of 50 dB. This design allows for a broad, mode-hop-free, single-frequency operation range (350 mA to 1000 mA). The DFB15TK laser system can also be tuned by adjusting the current or the temperature of the laser chip, with a mode-hop-free temperature tuning range of up to 2 nm.

These lasers are designed with laser frequency locking applications in mind. The systems come equipped with two analog current modulation ports (DC and AC coupled) and an analog temperature modulation port. These modulation inputs can be used to lock the laser frequencies to optical cavities or referenced using servo electronics in order to build stable laser systems.

The optical outputs of these lasers are fiber-coupled via FC/APC bulkhead (2.0 mm narrow key) output connectors. For best performance, we recommend connecting our PM FC/APC fiber patch cables that contain PM1550-XP fiber, such as the P3-1550PM-FC-1 patch cable. Note that the polarization axis is aligned with the narrow key of the bulkhead connector.

The systems are powered by a 12 VDC, 4 A input. A DS12 power supply is provided with each laser; see the Shipping List tab for a complete list of items included with the device.

Thorlabs also offers the DFB13TK as a single-frequency, low-noise laser system at 1310 nm, similar to the DFB15TK laser system.

Single-Frequency Operation and Wavelength Tuning
Factory-set for single-frequency operation, these lasers have a preset drive current and temperature. When the laser is enabled using the push-button switch on the front panel (see Front & Back Panels tab), the laser operates at the preset current level. Each laser is shipped with a test data sheet taken at the preset operating conditions; click here for a sample data sheet. The user can adjust the operating current either by sending a command through USB communication (see Remote Operation below) or by applying small modulation to the current through one of the modulation ports on the front panel of the system (see Analog Modulation below). Adjusting the operating current results in both a change in the output power but also a change in the laser wavelength. In addition to the operating current, the laser chip or FBG temperature is also adjustable either through the command-line interface (USB) or the front-panel modulation port. The temperature changes are primarily useful for tuning the laser wavelength while maintaining a relatively constant output power. See the Graphs and Specs tabs for more information on current and temperature tuning of these lasers.

Analog Modulation
By controlling the laser's drive current or temperature, the laser's wavelength can be tuned free of mode-hopping within a small range around its set-point. Three analog modulation ports (SMA Female) are included on the front panel (see the Front & Back Panels tab). Two of the ports allow the laser current to be modulated by ±10 mA around the current set point: of these, the port labelled "AC Current" can support modulation frequencies up to 20 MHz, and the port labelled "DC Current" accepts frequencies from DC to 5 MHz. The third port, which is labelled "Temp", controls the FBG temperature (ULN15TK) or chip temperature (DFB15TK) and accepts modulation frequencies from DC to 1 Hz. To see how the laser wavelength can be tuned around its set-point, please see the wavelength tuning plots provided in the Graphs tab.

Each modulation port accepts a -5 V to 5 V input signal. To convert the modulation voltage into a current or temperature change, see the Specs tab.

Remote Operation
Remote operation is also possible by connecting the laser to a PC (not included) via the USB port on the back of the unit and using a command line interface. In addition to basic functions such as turning the laser on and off or reading status indicators, the command line enables the user to modify the laser operating set-points. The full list of commands can be found in the laser manual. A USB-AB-72 USB 2.0 Type-A to Mini-B Cable is included with each device.

Adjusting the laser parameters (operating current and temperature) can be used for tuning the laser power or wavelength. Typical performance plots that demonstrate the laser's center wavelength tuning and the single-mode operating regions can be found on the Graphs tab. Please note that specific performance data is provided on the datasheet shipped with each device.

A 9-Pin female D-Sub connector is also included on the back panel of the system for RS-485 communication protocol. Please refer to the Pin Diagrams tab for the pin assignment of the connector.

Mounting
For mounting to an optical table, the compact housing of these benchtop lasers features four clearance holes each for M6 or 1/4" cap screws.

The plots below represent the relative intensity noise (RIN) and frequency noise for the low-noise, single-frequency laser systems.

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Example frequency noise data for the ULN15TK and DFB15TK laser systems operated at the factory-set conditions.

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The typical low-frequency relative intensity noise (RIN) for a ULN15TK and DFB15TK laser system operated at the factory-set conditions.

The plots below represent the typical performance of the ULN15TK ultra-low-noise laser system. Specific performance data is provided on the datasheet shipped with each device; click here for a sample data sheet.

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The typical single mode operating regions for a ULN15TK laser system operated at a Fiber Bragg Grating temperature (T_FBG) of 27 °C. Multimode regions have been removed.

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The center wavelength (λ_c) tuning with changes in the pump current and Fiber Bragg Grating temperature (T_FBG). Multimode regions have been removed.

The plots below represent the typical performance of the DFB15TK low-noise laser system. Specific performance data is provided on the datasheet shipped with each device; click here for a sample data sheet.

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Example output power vs. current data for a DFB15TK laser system operated at factory-set conditions.

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Example temperature and current tuning for a DFB15TK laser system operated at the factory-set conditions.

ULN15TK Pin Diagrams

DFB15TK Pin Diagrams

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Experimental Setup for Laser Stabilization

Laser Stabilization

The ULN15TK turnkey ultra-low noise laser offers exceptional optical spectral purity and is configured for out-of-the-box use with little setup effort. However, for the most demanding applications, such as optical clocks and high-resolution spectroscopy, active electronic feedback may be required to further suppress technical noise and drifts. This turnkey laser conveniently provides multiple high-bandwidth analog modulation ports for locking to external optical references such as an optical cavity or an atomic/molecular transition.

Thorlabs offers low loss supermirrors on their own or integrated into custom reference cavities. In the figure to the right, we show an example of how the Pound-Drever-Hall (PDH) locking scheme is used to lock the ULN15TK laser to a reference cavity comprised of XM12R8 and XM12P8 crystalline supermirrors attached to an ultra-low expansion glass spacer. The RF modulation input conveniently permits the PDH sidebands to be generated without an external optical modulator. Demodulation of the reflected signal from the cavity yields an error signal which can be processed with a servo filter to maintain resonance with the optical cavity. This turnkey laser provides a high-bandwidth DC-coupled current modulation port to correct for fast fluctuations, and a slower temperature modulation port with larger range to correct for slower drifts.

The ULN15TK and DFB15TK are both single-frequency turnkey laser systems that are equipped with low-noise drive electronics. Both systems enable laser frequency modulation, which can be utilized along with locking electronics to lock the laser frequency to optical references or cavities.

The ultra-low-noise (ULN) system provides exceptionally narrow Lorentzian linewidth (typ. 100 Hz) and record low RIN (typ. -160 dBc/Hz) for applications that require such demanding noise performance. The ULN design is based on a hybrid external-cavity semiconductor laser employing a fiber Bragg grating (FBG) for feedback to the laser cavity. As a result, the mode-hop-free single-frequency regions of the ULN are not continuous as the laser current is adjusted. Each mode-hop-free single-frequency region is typically limited to 20 mA to 50 mA of drive current (see Graphs tab for performance plots of the ULN15TK). The ULN wavelength can be adjusted via both current adjustment of the chip and temperature adjustment of the FBG. The broadest tuning range for a ULN is achieved via FBG temperature adjustment and is typically limited to <0.4 nm (with FBG temperature adjusted by approximately 40 °C). In order to maintain single-frequency operation as the temperature is tuned over a broad range, the laser current needs to be adjusted simultaneously.

The DFB15TK design is based on a distributed feedback (DFB) cavity on the laser chip. This design allows a much broader mode-hop-free single-frequency operation range (350 mA to 1000 mA). The Lorentzian linewidth of the laser is broader than the ULN with a typical value of 50 kHz, and the RIN is also higher with a typical value of -150 dBc/Hz. The linewidth and RIN of the DFB15TK are still exceptionally low for this laser type thanks to the low-noise electronics and turnkey system. The DFB can also be tuned in both ways by adjusting the current or the temperature of the laser chip. The broadest tuning can be achieved by adjusting the chip temperature and a typical tuning up to 2 nm (with chip temperature adjusted by 20 °C) can be achieved. Unlike the ULN, the DFB stays in single-frequency mode as the temperature is tuned without the need to simultaneously adjust the current.

In summary, the ULN offers superior phase noise (linewidth) and intensity noise (RIN) performance, while the DFB offers broader tuning and a broader mode-hop-free single-frequency operating region. The DFB is also a lower cost alternative to the ULN for applications that are less demanding on noise or linewidth.