MEMS-VCSEL Swept-Wavelength Laser Sources

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MEMS-VCSEL Tutorial
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OCT Applications Team Based in Lübeck, Germany

We are happy to assist with purchasing or information requests. You can easily contact us directly at oct@thorlabs.com or via our online request form; a Thorlabs customer representative will contact you shortly.

Common Specifications^a
Wavelength Sweep Range^b	100 nm
Coherence Length	≥100 mm
Ripple Noise Suppression	≥50 dB
Relative Intensity Noise (RIN)	≤2%

These are typical values. See the Specs tab for details.
The optical bandwidth that can be sampled using the provided k-clock is ≥95 nm.

Features

Benchtop Swept-Wavelength Laser Sources
1060 nm or 1300 nm Center Wavelength
100 nm Mode-Hop-Free Sweep Range
User-Adjustable k-Clock Signal Delay
Trigger and k-Clock Signal Output Provided

Thorlabs' Swept-Wavelength Laser Sources are available with 1060 nm or 1300 nm center wavelengths and operate mode-hop-free over the full 100 nm wavelength range. They have record-breaking coherence lengths of over 100 mm. Options are available for sweep rates from 50 kHz to 400 kHz and MZI delays from 8 mm to 72 mm. A Thorlabs balanced detector can also be integrated into the benchtop source. If the options desired are not covered by the configurations available below, please contact our MEMS-VCSEL Source Team for custom configurations.

These single mode benchtop laser sources are designed primarily for high-speed and long-range optical coherence tomography (OCT) systems requiring superior sensitivity, as well as being well-suited for metrology, spectroscopy, and other applications. The sources are based on a patented microelectromechanical system (MEMS) tunable vertical cavity surface emitting laser (VCSEL) and include an active power control that maintains constant output power over the lifetime of the laser. The FC/APC fiber-coupled laser output port can be closed by sliding the door to the right when not in use, serving as both a beam block and a dust cover.

All drive electronics and trigger signals needed to easily integrate our MEMS-VCSEL laser into custom swept-source OCT systems are provided. An output digital “k-clock” signal generated by the integrated Mach-Zehnder interferometer (MZI) and drive electronics can be used as a data acquisition sampling clock, with no further resampling in k-space required. The k-clock signal delay can be adjusted up to ±8 ns, and the output power of the laser up to a maximum of ±5% by the user.

OCT Imaging with a MEMS-VCSEL Swept Source
OCT systems built using our MEMS-VCSEL sources provide high-quality images and are capable of significant imaging depths. The OCT cross-sectional image of the anterior eye shown below (left) was obtained using the equivalent of an SL131090 integrated into a Vega OCT system. The laser was swept over a wavelength range of 100 nm and had a 100 kHz sweep rate. This Vega system had a 44 mm Mach-Zehnder interferometer delay, an axial resolution in air better than 16 μm, and was capable of high-resolution imaging to a maximum depth of 11 mm. The OCT measurement signal was digitized with a 500 MSPS A/D converter, and the system provided 100 000 A-scans/s. Details of this system and its performance can be found here.

Point spread function (PSF) is a measurement of the optical performance of an imaging system, and narrower peaks indicate better performance. The PSF shown below (center) was obtained for the same Vega system as was used for the image to the left, but is typical of any OCT system using one of our swept-source lasers. For example, a system with an integrated SL101081 is shown to the right with similar performance, but with a maximum imaging depth of 6 mm.

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OCT Cross-Sectional Image of the Anterior Eye, from an OCT System
with Integrated SL13 MEMS-VCSEL Swept Source

MEMS-VCSEL Point Source Function, SL13 Series

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Click Here to Download the Raw Data
PSF Obtained for an OCT System with Integrated SL13 Series
MEMS-VCSEL Swept Source, Using Integrated k-Clock

MEMS-VCSEL Point Source Function, SL10 Series

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Click Here to Download the Raw Data
PSF Obtained for an OCT System with Integrated SL10 Series
MEMS-VCSEL Swept Source, Using Integrated k-Clock

1060 nm MEMS-VCSEL Swept Sources

Item #s	SL100060, SL100061	SL101080, SL101081	SL101060, SL101061	SL102080, SL102081	SL104070, SL104071
Sweep Rate	60 kHz	100 kHz	100 kHz	200 kHz	400 kHz
Duty Cycle (Unidirectional Sweep)	>60%	>60%	>60%	>50%	>50%
MZI Delay	48 mm	24 mm	48 mm	24 mm	8 mm
k-Clock Max Frequency	500 MHz	500 MHz	900 MHz	900 MHz	900 MHz
Imaging Depth Range (Approximate)	12 mm	6 mm	12 mm	6 mm	2 mm

Optical Specifications for 1060 nm Swept Sources
Parameter	Symbol	Minimum	Typical	Maximum
Center Wavelength	λ_c	1040 nm	1060 nm	1080 nm
Wavelength Sweep Range (-15 dB)	Δλ	95 nm^a	100 nm	N/A
Average Output Power	P_o	15 mW	17.5 mW	20 mW
Coherence Length	L_Coh	100 mm	>1 m	-
Spectral Ripple Noise Suppression	dP	-	≥50 dB	-
Relative Intensity Noise	RIN_ORTHO	-	-	2%
λ Trigger Wavelength	-	1055 nm	1060 nm	1065 nm
Output Fiber Numerical Aperture	NA	-	0.105	-
Laser Classification (IEC 60825-1:2014)	-	3R

The optical bandwidth that can be sampled using the provided k-clock is ≥95 nm.

1300 nm MEMS-VCSEL Swept Sources

Item #s	SL130160, SL130161	SL131090, SL131091	SL131160, SL131161	SL132120, SL132121	SL134050, SL134051
Sweep Rate	50 kHz	100 kHz	100 kHz	200 kHz	400 kHz
Duty Cycle (Unidirectional Sweep)	>60%	>60%	>60%	>50%	>50%
MZI Delay	72 mm	44 mm	72 mm	32 mm	12 mm
k-Clock Max Frequency	500 MHz	500 MHz	900 MHz	900 MHz	900 MHz
Imaging Depth Range (Approximate)	18 mm	11 mm	18 mm	8 mm	3 mm

Optical Specifications for 1300 nm Swept Sources
Optical Specifications	Symbol	Minimum	Typical	Maximum
Center Wavelength	λ_c	1280 nm	1300 nm	1320 nm
Wavelength Sweep Range (-10 dB)	Δλ	95 nm^a	100 nm	N/A
Average Output Power	P_o	20 mW	25 mW	50 mW
Coherence Length	L_Coh	100 mm	>1 m	-
Spectral Ripple Noise Suppression	dP	-	≥50 dB	-
Relative Intensity Noise	RIN_ORTHO	-	-	2%
λ Trigger Wavelength	-	1305 nm	1310 nm	1315 nm
Output Fiber Numerical Aperture	NA	-	0.09	-
Laser Classification (IEC 60825-1:2014)	-	1M

The optical bandwidth that can be sampled using the provided k-clock is ≥95 nm.

Common Specifications, All MEMS-VCSEL Swept Sources

Physical Specifications
Parameter	Minimum	Maximum
Main AC Voltage	100 VAC	250 VAC
Power Consumption	-	45 W
Line Frequency	50 Hz	60 Hz
Operating Temperature	10 °C	35 °C
Storage Temperature	0 °C	50 °C
Relative Humidity	-	85%, Non-Condensing

Electrical Performance Specifications
Parameter	Minimum	Typical	Maximum
k-Clock "Dummy Clock" Frequency	-	250	MHz
k-Clock Amplitude	-	-	1.0 V_pp
Sweep Trigger Pulse Width (100 kHz Sweep Rate)	-	350 ns	-
Sweep Trigger Rise Time	-	0.6 ns	-
Sweep Trigger Output Type	LVTTL
λ Trigger Pulse Width (100 kHz Sweep Rate)	25 ns	50 ns	75 ns
λ Trigger Rise Time	-	0.6 ns	-
λ Trigger Output Type	LVTTL
Laser Controller Start Delay	5 s	-	15 s

Click to Enlarge
Click Here to Download the Raw Data
Optical Output Spectrum of SL10xxxx Swept Source

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Click Here to Download the Raw Data
Optical Output Spectrum of SL13xxxx Swept Source

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Schematic of MEMS-VCSEL Swept Source with Balanced Detection

Thorlabs' MEMS-VCSEL benchtop systems incorporate all the necessary drive electronics, temperature controllers, trigger signals, and optical isolators for easy operation and integration into any swept-source OCT system. Additionally, these benchtop laser sources utilize a specially designed Mach-Zehnder Interferometer (MZI) "k-clock" that provides a digital output signal for triggering data acquisition.

The figure to the right shows a schematic of the MEMS-VCSEL benchtop system. These systems consist of a MEMS-VCSEL cavity, a booster optical amplifier (BOA), a fiber-optic monitoring network, and signal generation circuits. The optical output of the MEMS-VCSEL cavity module is connected to the optical input of the BOA. Integrated optical isolators in the benchtop laser sources eliminate the need for additional isolators external to the laser. Three electrical output signals, shown at the bottom of the schematic, can be used via SMA connectors to synchronize a data acquisition system to the swept optical output.

Sweep Trigger
The Sweep Trigger provides an electronically generated line trigger at the beginning of the sweep. The rising edge of the pulse is synchronized to the beginning of the sweep. The sweep trigger is generally used as a trigger for data acquisition systems. The signal is series terminated with 50 Ω.

Wavelength Trigger (λ Trigger)
"λ Trigger" is an optically generated line trigger that indicates when the laser output has crossed a certain wavelength. This fiber-Bragg-grating-based optical trigger usually represents the middle of the sweep. Due to its higher wavelength stability, it is used as an alternative to the sweep trigger and can be used for phase-sensitive applications where sweep-to-sweep stability is required. The signal is series terminated with 50 Ω.

k-Clock
The k-clock signal is used as a data acquisition sampling clock, internally generated from an MZI. The digital output from the MZI "k-Clock" is linear in wavenumber, allowing optically generated signals to be sampled linearly in "k-space," rather than in the time domain. Electrically, the signal is an AC-coupled digital clock with an amplitude between 0.7 and 1.0 Vpp. During the back-scan, the k-clock circuit will switch over to a "dummy clock", which operates at a fixed frequency and is not correlated to the optical subsystem. The dummy clock runs at approximately 250 MHz.

Balanced Detector (Not Included with SL1xxxx0 Sources)
The balanced detector (included with item #s that end in 1) consists of two well-matched photodiodes and an ultra-low-noise, high-speed transimpedance amplifier. This acts as a balanced receiver by subtracting Rx+ from Rx-, resulting in the cancellation of common mode noise. This produces an RF Output voltage that has small changes in the signal path extracted from the interfering noise floor. The optical input power level to each photodiode can be monitored through the Monitor+ and Monitor- fast monitor outputs.

*The balanced detector and its associated connections will not be present on item #s ending in 0.

Front and Back Panels (without Balanced Detector, SL1xxxx0 Sources)

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Front Panel of MEMS-VCSEL Source

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Back Panel of MEMS-VCSEL Source

Front and Back Panels (with Balanced Detector, SL1xxxx1 Sources)

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Front Panel of MEMS-VCSEL Source with Balanced Detector

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Back Panel of MEMS-VCSEL Source with Balanced Detector

Front Panel
Callout	Description
F1	Laser Enable Button
F2	Laser Power Key Switch
F3^a	Optical Output, FC/APC Connector
F4^a	Rx+ Optical Input, FC/APC Connector
F5^a	Rx- Optical Input, FC/APC Connector

Slide the door to the right to protect from dust and exposure; slide to the left to open.

Back Panel
Callout	Description
B1	Data Acquisition (DAQ) Trigger Signal
B2	Center Wavelength (CWL) Trigger Signal
B3	k-Clock Signal
B4	Balanced Detector RF Output Signal
B5	Balanced Detector Monitor+
B6	Balanced Detector Monitor-
B7	USB Type B Port
B8	Main Power Switch
B9	Fuse Holder
B10	AC Power Cord Connector
B11	Interlock Connector

Figure1: VCSEL Schematic
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Figure 1: Praevium's MEMS-Tunable VCSEL is an innovative design that offers high-speed and broadband emission with long coherence length. This is an ideal combination for an OCT swept laser source.

Figure2: MEMS-VCSEL Wafer Photo
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Figure 2: MEMS-tunable VCSELs can be densely packed on a single wafer to increase the potential yield. The inset shows a single MEMS-tunable VCSEL device after fabrication. The overall size of the MEMS-tunable VCSEL is approximately 600 µm x 600 µm square.

VCSEL Overview
Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor-based devices that emit light perpendicular to the chip surface, as shown in Figure 1. VCSELs were originally developed as low-cost, low-power alternatives to edge-emitting diodes, mainly for high-volume datacom applications. Quickly thereafter, the advantages of VCSELs became evident, leading them to being preferred light sources over edge-emitters in many applications. Compared to edge-emitting sources, VCSELs offer superior output beam quality and single mode operation.

MEMS-tunable VCSELs utilize microelectromechanical mirror systems (MEMS) to vary the cavity length of the laser, thereby tuning the output wavelength. MEMS-tunable VCSELs have existed for several years; however, the limited tuning range and output power of these devices have precluded them from being used in OCT applications. Praevium Research, in cooperation with Thorlabs and MIT, has since developed a MEMS-tunable VSCEL design that overcomes these previous limitations.

In order for a MEMS-tunable VCSEL to be successful for applications in OCT, it needs to meet certain standards:

Rapid Sweep Speed
Broad Tuning Range
Long Coherence Length
High Laser Output Power

Rapid Sweep Speed
Applications using OCT demand high-speed imaging without sacrificing imaging quality. Fast imaging rates allow better time resolution, dense collection of 3D datasets, and decreased laser exposure times to the sample.

Currently, there exist a few swept-source lasers that offer high-speed scanning. Fourier domain mode-locked lasers, for example, achieve extremely high imaging speeds but require the use of very long fiber optic delays in the laser cavity and can only operate in wavelength ranges where the fiber loss is low. Of the commercially available high-speed swept lasers, many operate with multiple longitudinal modes or have long cavity lengths, which limit coherence length or tuning speed, respectively.

The low mass of the MEMS-tuning mirror in a MEMS-based tunable VCSEL and the short cavity length both contribute to its high-speed operation. The short cavity length also places only one mode in the gain spectrum, enabling single-mode continuous operation. We have recently measured greater than 500 kHz sweep rates using a MEMS-tunable VCSEL prototype, without using optical multiplexing to increase the sweep speed.

Broad Tuning Range
High-resolution imaging depends on the overall tuning bandwidth of the swept-source laser. Praevium boasts the broadest bandwidth MEMS-tunable VCSEL that has ever been developed. A unique design incorporating broadband, fully oxidized mirrors, as well as wideband gain regions and thin active regions, has currently resulted in greater than 100 nm of continuous mode-hop-free tuning, centered around 1300 nm. For details, please see Figure 3.

Figure 3: Spectral Tuning
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Figure 3: MEMS-tunable VCSELs are capable of tuning over 100 nm. Here we show single-mode operation over a 110 nm spectral tuning range centered at 1300 nm.

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Figure 4: Spectrum of MEMS-tunable VCSEL operating at 200 kHz, with a center wavelength around 1310 nm, and post amplification using a BOA.

Long Coherence Length
A significant limitation to most OCT systems is the depth of view (maximum imaging depth range). Especially in clinical applications, where sample thickness, patient motion, and sample location cannot be controlled, a long depth of view is advantageous. A long coherence length alone, however, is not enough. Image sensitivity needs to be virtually unaffected throughout the entire depth. Due to the micron-scale cavity length of the VCSEL and single mode, mode-hop-free operation, we have measured coherence lengths of greater than 100 mm from our MEMS-tunable VCSEL with nearly no signal degradation. Currently limited by detector bandwidth, we are confident that the MEMS-tunable VCSEL is able to achieve even longer imaging depths than have been measured to date. This remarkable depth of view will not only benefit the medical imaging community but also open doors to other applications such as large objective surface profiling, fast frequency domain reflectometry, and fast spectroscopic measurements with high spectral resolution.

High Output Power
Increased imaging speed often comes at the cost of decreased output power and/or optical power on the sample. One advantage of edge-emitting light sources over VCSELs is that they can emit greater output powers. As a general rule, most OCT imaging applications need a minimum of 20 mW of laser output power to maintain image quality when operating at faster scan rates. To reach this goal, the MEMS-tunable VCSEL is coupled with a booster optical amplifier (BOA) to achieve greater than 25 mW of power. An additional advantage of this post-amplification scheme is that the BOA reshapes the MEMS-VCSEL output spectrum such that it is much more uniform.

Additional Considerations and Manufacturing Capabilities
A special feature of the MEMS-tunable VCSEL is that it is scalable for different wavelengths. Through innovative combinations of gain materials and dielectric mirrors, a wide wavelength range in the visible or near infrared can be reached, enabling expansion of this new family of light sources.

As we further develop this light source, we look forward to finding new and exciting applications for its use. Please contact us to discuss how a MEMS-tunable VCSEL may advance your research.

Fabrication of a MEMS-Tunable VCSEL

Step 1
Step 1
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The VCSEL wafer begins with a multiple quantum well (MQW) active region (A) that is grown on an InP substrate (B) and bonded to a GaAs-based mirror (C) grown on a GaAs substrate (D).

Step 2
Step 2
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The InP substrate is chemically etched down to a strategically located stop-etch layer (E). The GaAs-based mirror is oxidized to create a wideband dielectric mirror (F).

Step 3
Step 3
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After removal of the stop-etch layer, an AR coating (G) and annular MEMS bottom actuator contact (H) are deposited on top of the MQW active region.

Step 4
Step 4
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A sacrificial layer (I) of a specifically designed thickness and composition is deposited.

Step 5
Step 5
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A membrane layer (J) and annular top MEMS actuator contact (K) are deposited on top of the sacrificial layer.

Step 6
Step 6
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Finally, a dielectric mirror (L) is deposited and patterned. The top MEMS contact is further patterned to complete creation of the actuator. The sacrificial layer is undercut to leave a suspended, moveable top mirror above the MQW structure, producing a VCSEL with a MEMS-based tuning element in a single device.

Laser Safety and Classification

Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina.

Safe Practices and Light Safety Accessories

Thorlabs recommends the use of safety eyewear whenever working with laser beams with non-negligible powers (i.e., > Class 1) since metallic tools such as screwdrivers can accidentally redirect a beam.
Laser goggles designed for specific wavelengths should be clearly available near laser setups to protect the wearer from unintentional laser reflections.
Goggles are marked with the wavelength range over which protection is afforded and the minimum optical density within that range.
Laser Safety Curtains and Laser Safety Fabric shield other parts of the lab from high energy lasers.
Blackout Materials can prevent direct or reflected light from leaving the experimental setup area.
Thorlabs' Enclosure Systems can be used to contain optical setups to isolate or minimize laser hazards.
A fiber-pigtailed laser should always be turned off before connecting it to or disconnecting it from another fiber, especially when the laser is at power levels above 10 mW.
All beams should be terminated at the edge of the table, and laboratory doors should be closed whenever a laser is in use.
Do not place laser beams at eye level.
Carry out experiments on an optical table such that all laser beams travel horizontally.
Remove unnecessary reflective items such as reflective jewelry (e.g., rings, watches, etc.) while working near the beam path.
Be aware that lenses and other optical devices may reflect a portion of the incident beam from the front or rear surface.
Operate a laser at the minimum power necessary for any operation.
If possible, reduce the output power of a laser during alignment procedures.
Use beam shutters and filters to reduce the beam power.
Post appropriate warning signs or labels near laser setups or rooms.
Use a laser sign with a lightbox if operating Class 3R or 4 lasers (i.e., lasers requiring the use of a safety interlock).
Do not use Laser Viewing Cards in place of a proper Beam Trap.

Laser Classification

Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below:

Class	Description	Warning Label
1	This class of laser is safe under all conditions of normal use, including use with optical instruments for intrabeam viewing. Lasers in this class do not emit radiation at levels that may cause injury during normal operation, and therefore the maximum permissible exposure (MPE) cannot be exceeded. Class 1 lasers can also include enclosed, high-power lasers where exposure to the radiation is not possible without opening or shutting down the laser.
1M	Class 1M lasers are safe except when used in conjunction with optical components such as telescopes and microscopes. Lasers belonging to this class emit large-diameter or divergent beams, and the MPE cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. However, if the beam is refocused, the hazard may be increased and the class may be changed accordingly.
2	Class 2 lasers, which are limited to 1 mW of visible continuous-wave radiation, are safe because the blink reflex will limit the exposure in the eye to 0.25 seconds. This category only applies to visible radiation (400 - 700 nm).
2M	Because of the blink reflex, this class of laser is classified as safe as long as the beam is not viewed through optical instruments. This laser class also applies to larger-diameter or diverging laser beams.
3R	Lasers in this class are considered safe as long as they are handled with restricted beam viewing. The MPE can be exceeded with this class of laser, however, this presents a low risk level to injury. Visible, continuous-wave lasers are limited to 5 mW of output power in this class.
3B	Class 3B lasers are hazardous to the eye if exposed directly. However, diffuse reflections are not harmful. Safe handling of devices in this class includes wearing protective eyewear where direct viewing of the laser beam may occur. In addition, laser safety signs lightboxes should be used with lasers that require a safety interlock so that the laser cannot be used without the safety light turning on. Class-3B lasers must be equipped with a key switch and a safety interlock.
4	This class of laser may cause damage to the skin, and also to the eye, even from the viewing of diffuse reflections. These hazards may also apply to indirect or non-specular reflections of the beam, even from apparently matte surfaces. Great care must be taken when handling these lasers. They also represent a fire risk, because they may ignite combustible material. Class 4 lasers must be equipped with a key switch and a safety interlock.
All class 2 lasers (and higher) must display, in addition to the corresponding sign above, this triangular warning sign

Posted Comments:

nyonuimon (posted 2018-09-04 18:57:59.367)

Hello, This is Yeon Hee Chang I am researcher of DGIST, South Korea. I am very interested in the laser source, SL132120 - MEMS-VCSEL Swept Source, 1300 nm, 200 kHz. I want to get a formal quotation of this product. Is it available? Please send the quotation to my e-mail. Thank you :) Best regards From Yeon Hee Chang

llamb (posted 2018-09-05 08:17:04.0)

Hello Yeon. For a formal quote or other questions on our MEMS-VCSEL Swept-Wavelength Laser Source, you may email OCT@thorlabs.com. We will have a representative reach out to you by email in this case.

thomas.juhasz (posted 2018-09-03 14:29:07.34)

Hello, I have a very basic question for the MEMS-VCSEL Swept-Wavelength Laser Source: has the source a static mode, where the wavelength is kept constant, or is the wavelength swept continuously? If there is a static mode, what is the wavelength accuracy and spectral linewidth for different wavelengths within the tuning range? Additionally, the output power range from min 20 to max 50mW seems rahther large. Is a preselection with respect to the output power possible? Thanks and best regards Thomas

YLohia (posted 2018-09-24 11:00:16.0)

Hello Thomas, thank you for contacting Thorlabs. The wavelength will be swept continuously for the entire range -- unfortunately, there is no static mode and there is no preselection with respect to output power. Our OCT team (oct@thorlabs.com) will reach out to you directly to discuss your application in greater detail.

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