Fast Laser Power Modulators for Two-Photon Microscopy

Pockels Cell and Acousto-Optic Modulator Alternative
Broad Wavelength Coverage from 700 nm to 1350 nm Available
Photoactivation Control Up to 500 nJ
Near-Zero Dispersion Reduces Need for Pre-Compensation

OM6ENH

>2500:1 Contrast Ratio,
700 - 1350 nm

Two-channel 2P image of mouse kidney cells acquired with Thorlabs' Bergamo^® III microscope using the OM6N fast laser power modulator to define a region of interest.

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Overview
Specs
Graphs
Beam Quality
Side Panel
Pin Diagrams
Feedback

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Figure 1.1 Typical transmissions for the OM6N (solid), OM6NH (dotted), and OM6ENH (dashed) modulators at 700 nm (blue traces), 1100 nm (red traces), and 1300 nm (green trace). For each unit, the full transmission (at 0 V) differs with wavelength due to the reflective nature of the gold-coated mirrors. Additional performance graphs are available on the Graphs tab.

Applications

Fast Power Modulation of NIR Femtosecond Lasers
Two-Photon Fluorescence Microscopy
Photoactivation
Galvo-Galvo or Galvo-Resonant Scanning
Edge and Frame Blanking
Region of Interest Selection

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Figure 1.2 Our fast laser power optical modulator shown with the included photodetector removed, providing access to four 4-40 taps for 30 mm cage system compatibility.

Features

Offers DC Power Attenuation and High-Speed Modulation
Full Extinction AC Modulation Up to 200 kHz
Supports Region of Interest Selection, Edge Blanking, and Frame Blanking in Imaging Systems
Pockels Cell Alternative with Polarization-Independent Performance
Intelligent Electronic Driving for Smooth Optical Response and Arbitrary Trajectory Tracking
700 - 1100 nm or 700 - 1350 nm Operating Ranges
<1 µs On/Off Time
Contrast Ratios Up to 2500:1
Compatible with ThorImage^®LS and ScanImage Software
Photodetector Included for Monitoring/Calibrating Output Power
Near-Zero (<100 fs²) Dispersion
Passively Cooled to Eliminate Vibrations

Thorlabs' Fast Laser Power Modulators use deformable mirror technology to provide high-speed intensity modulation and beam attenuation. The all-reflective design incorporates gold-coated MEMS mirrors that are polarization independent and introduce minimal (<100 fs²) dispersion. This unidirectional device is designed for a collimated, 1 to 2 mm (1/e²) diameter input beam with up to 10 W of optical power and up to 500 nJ of pulse energy, enabling use with popular 1 µm high-pulse-energy photoactivation lasers. These modulators are available with either standard (>250:1) or high (>2500:1) contrast ratios for wavelength ranges of 700 - 1100 nm or 700 - 1350 nm (high contrast only).

Transmission, which is a function of both input voltage and wavelength as shown in Figure 1.1, is controlled by a 0 to 1 V analog input voltage; this voltage can be easily generated by off-the-shelf data acquisition (DAQ) hardware or other electronics. The device supports continuous DC operation as well as modulation frequencies up to 200 kHz, making it ideal for imaging applications such as fly-back blanking or region of interest (ROI) selection.

The higher contrast achieved with the OM6NH(/M) and OM6ENH(/M) modulators is made possible by incorporating two MEMS mirrors, the deformation of which behaves like a grating, into the design; one diffracts horizontally and the other vertically to provide enhanced contrast performance.

Ideal for Photoactivation and Two-Photon Fluorescence Applications
These modulators are suitable for many applications including two-photon fluorescence microscopy and photoactivation. Both of these techniques require fast laser power modulation of ultrafast NIR laser pulses to limit sample exposure during imaging and reduce thermally induced damage. High-speed modulation is traditionally accomplished with bulk crystal modulators, such as Pockels cells and acousto-optic modulators (AOMs). As they rely on bulk materials, they introduce significant dispersion, degrading image quality.

Replaceable Photodetector
An internal beam sampler directs a small portion (<1%) of the modulated beam to a separate output port where a photodetector can be used to monitor the transmission. If desired, the included, pre-installed detector can be easily removed and replaced with a user-supplied detector; to do so, loosen the 8-32 mounting screw located on the underside of the modulator using a 9/64" hex key. The OM6N(/M) and OM6NH(/M) modulators each include a PDA100A2 detector, while the OM6ENH(/M) modulator includes a PDA20C2 detector.

Housing and Mounting Features
The input and output ports of each laser power modulator feature internal SM05 (0.535"-40) threading, which can be used to install external SM05-threaded alignment targets or irises (not included). All three ports also have 4-40 threaded holes for compatibility with our 30 mm cage systems. To aid in initial alignment, we recommend using an SM05 slotted lens tube or a 30 mm cage system assembly on the modulator input.

Three slots in the enclosure allow the modulator to be mounted directly to an optical table with table clamps, such as our CL6 clamps (not included), resulting in a minimum beam height of 1.25". The base also includes three 8-32 (M4) mounting holes for attaching pedestal posts, which can be used to elevate the modulator to the desired beam height.

The unit is shipped with a 24 V power supply with an KPJX-4S-S connector for the modulator, as well as a ±12 V supply for the photodetector. For more information, please see the Pin Diagrams tab.

Figure 2.1 Laser Power Modulator Housing Dimensions

Item #	OM6N(/M)	OM6NH(/M)	OM6ENH(/M)
Contrast Ratio	>250:1	>2500:1	>2500:1
Operating Range^a	700 - 1100 nm		700 - 1350 nm
Included Photodetector	PDA100A2		PDA20CS2
Input Polarization	Any
Rise/Fall Time^b	<1 µs
Modulation Frequency^c	0 - 200 kHz
Small Signal Modulation Bandwidth (-3 dB)	>1 MHz
Insertion Delay^d	<5 µs
Modulation Voltage^e	0 - 1 V
Input Impedance^e	10 kΩ
Transmission (@ 800 nm)	>80%
Input Pulse Energy	500 nJ (Max)
Input Optical Power	10 W (Max)
Internal Aperture	Ø3.8 mm
Recommended Collimated Input Beam Diameter (1/e²)	Ø1 - 2 mm
Internal Optical Path Length	100 cm
Dispersion	<100 fs²
Transmitted Wavefront Error^f	<λ/4 (at 633 nm)
Power Supply	24 V, 1 A
Power Connector	R7B Female Connector
Operating Temperature	15 - 35 °C
Operating Humidity	20 - 60%
Storage Temperature	-25 - 70 °C
Housing Dimensions (L x W x H)	11.45" x 6.83" x 2.80" (290.8 mm x 173.4 mm x 71.1 mm)

The laser power modulator can be used outside the wavelength range. However, the damage threshold for wavelengths outside this range is unspecified and may be lower, especially for shorter wavelengths. The modulator may not be able to achieve full extinction for wavelengths higher than the max operating wavelength.
The time required for the transmission to rise from 10% to 90% or to fall from 90% to 10%. Response depends on DC attenuation level.
Modulation frequency is defined as the frequency range over which the maximum extinction of the beam is attained with full modulation. Frequencies higher than 200 kHz can be used to drive the modulator, at reduced contrast ratios.
Insertion delay is a fixed time delay that the modulator introduces between the optical response and the modulation input signal.
A 10 kΩ internal pull-down resistor holds the input voltage at 0 V if no input is provided.
Independent of modulation level.

Performance Graphs

This data reflects the typical performance of our high-speed optical modulators and is presented for reference only. Actual performance will vary from device to device. The guaranteed specifications are shown in the Specs tab. Please note that the maximum contrast ratio, defined as maximum transmission to minimum transmission, can only be achieved when the extinction voltage is carefully calibrated.

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Typical transmission for the OM6N(/M) modulator.

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Typical transmission for the OM6NH(/M) modulator.

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Typical transmission for the OM6ENH(/M) modulator.

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Typical extinction voltages as a function of input wavelength. For each unit, the extinction voltage is the input voltage resulting in minimum transmission. The exact voltage for maximum extinction varies from device to device. This curve is only applicable over the wavelength range of the device.

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Plot showing the high-frequency temporal response of OM6 Series
Optical Modulators.

Beam Quality for the OM6 Modulators

In designing Thorlabs' OM6 Series Fast Laser Power Modulators, reliably maintaining output beam quality was a priority. This is because a beam's mode quality is important for a wide range of applications, including telecommunications and two-photon microscopy.

These modulators are polarization independent and have no significant impact on the transmitted beam profile, as shown in Figures 4.1, 4.2, 4.3, and 4.4, which were acquired using the previous generation BC106N-VIS CCD Camera Beam Profiler, the S1FC1060 1064 nm Laser Source, and the OM6N Modulator. Figure 4.1 shows a beam profile before entering an OM6 modulator. Figures 4.2, 4.3, and 4.4 show the beam profile after being modulated. This data is representative of all OM6 Series Modulators.

Before Modulator

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Figure 4.1 Beam profile before reaching a OM6 modulator.

Full Transmission

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Figure 4.2 Beam profile after traveling through a OM6 modulator with full transmission.

20% Transmission

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Figure 4.3 Beam profile after traveling through a OM6 modulator with 20% transmission.

10% Transmission

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Figure 4.4 Beam profile after traveling through a OM6 modulator with 10% transmission.

Fast Laser Power Modulator Side Panel
Callout	Description
1	0 to 1 V Modulation Voltage (BNC)
2	DC Power Switch
3	LED Power Indicator
4	DC Power Cord Connector for Modulator (R7B)
5	DC Power Cord Connector for Photodetector
6	0 to 10 V Photodetector Output (BNC)

Laser Power Modulator Electrical Connections

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Figure 5.1 High-Speed Optical Modulator Electrical Connections

OM6 Series Fast Laser Power Modulator Connections

BNC Female, Modulation Signal Input

Modulation Signal Input is 0 - 1 V

KPJX-4S-S Connector, Female Power IN

Pinout for KPJX-4S-S Connector

PDA100A2 & PDA20CS2 Photodetector Connections

BNC Female Output (Photodetector)

0 - 10 V Output

Male (Power Cable)*

*For the Included LDS12B Power Supply

Female Power IN (Photodetector)

Pinout for PDA Power Connector

Posted Comments:

Yaqing Zhang (posted 2021-07-03 03:41:25.42)

Hi, Could you please provide the optical path length from the input aperture to output aperture of the fast modulator (OM6NH)? The specs says the internal optical path length is 100mm, but the housing length is already 290.83 mm, so I believe the optical path from the two apertures must be larger than 290.83mm. Thanks!

YLohia (posted 2021-07-12 10:36:04.0)

Hello, thank you for bringing this typo to our attention. The path length is approximately 100 cm, not 100 mm. This will be corrected on our web documentation shortly.