Features

Constant Magnification Over Entire Field of View (FOV)
Constant Spot Size
Flat Image Plane
Excellent Coupling Efficiency
Large Field of View
Dispersion Compensators Available for LSM02, LSM03 and LSM04.

The LSM series of scan lenses are telecentric objectives that are ideal for use in laser scanning applications like Optical Coherence Tomography (OCT). Telecentric objectives are used in OCT and other laser imaging systems because of the advantages of a flat imaging plane when used in applications that scan the laser across the sample being imaged. A flat imaging plane minimizes image distortion, which in turn allows for the creation of geometrically correct images without the need for extensive post image processing. A telecentric scan lens also maximizes the coupling of the light scattered or emitted from the sample (the signal) into the detection system. In addition, the spot size in the image plane is nearly constant over the entire FOV (See Scan Lens Plots tab) so that resolution of the image is constant.

The LSM02, LSM03, and LSM04 scan lenses have an AR coating designed to minimize back reflections from broadband light sources with a central wavelength of 1315 nm (see plot below), which is a popular wavelength for OCT systems. For wavelengths outside of the effective wavelength range of these lenses, the LSM02-BB, LSM03-BB, and LSM04-BB scan lenses were added to the product line. The -BB scan lenses have an AR coating that is effective over a wavelength range from 800 nm to 1100 nm. In order to assist with the integration of the -BB scan lenses into OCT systems, several system parameters have been specified (see the Scan Lens Specs tab) at 850 nm and 1050 nm. However, the -BB scan lenses can be used with light sources throughout the 800 nm to 1100 nm range.

Dispersion compensating blocks have also been added to our product line to help compensate for dispersion up to second order. These blocks also have an AR coating for the 800 - 1400 nm wavelength range. Please contact our Technical Support Department if you need additional support information.

OCT scan lens application schematic

The LSM Scan Lens, when used as depicted in the schematic, not only focuses the laser on the sample in the Field of View (FOV) but it also collects the light from the spot being imaged in the FOV so that it can be coupled back into the optical fiber.

Items	Threading*	Thread Length (A)^†	Barrel Diameter	Length of Barrel (L)^†
LSM02 and LSM02-BB	M25 x 0.75	4.9 mm (0.19")	33 mm (1.30")	23.2 mm (0.91")
LSM03 and LSM03-BB	M25 x 0.75	4.5 mm (0.18")	34 mm (1.35")	25.5 mm (1.00")
LSM04 and LSM04-BB	M25 x 0.75	4.7 mm (0.19")	34 mm (1.35")	38.25 mm (1.51")

* An SM1A12 adapter can be used to mate the LSM scan lenses to standard SM1 threaded Thorlabs components. Using an SM1A12 adapter and an SM1A4 adapter in combination, the scan lens can be used with RMS threaded components.
^† See schematic above for the labeled dimension.

Broadband OCT Scan Lenses

Item #	LSM02-BB		LSM03-BB		LSM04-BB
Magnification	10X		5X		3X
Design Wavelengths	850 nm	1050 nm	850 nm	1050 nm	850 nm	1050 nm
Wavelength Range	±40 nm	±50 nm	±40 nm	±50 nm	±40 nm	±50 nm
Effective Focal Length (EFL)¹	17.93 mm	17.97 mm	35.78 mm	35.88 mm	53.61 mm	53.79 mm
Lens Working Distance (LWD)	7.5 mm	7.5 mm	25.1 mm	25.0 mm	42.3 mm	42.2 mm
Scanning Distance (SD)(Distance from Pupil Position to Mounting Plane)	15 mm
Pupil Size (1/e²) (EP)	4 mm
Depth of View (DOV)	0.12 mm		0.58 mm		1.15 mm
Variation of Spot Size Over FOV (dS)	8 µm	9 µm	3 µm	3.5 µm	8 µm	9 µm
Distance Between First Lens Surface and Mounting Plate	-4.0 mm		-3.1 mm		-3.0 mm
Field of View (FOV)	4.7 x 4.7 mm		9.4 x 9.4 mm		14.1 x 14.1 mm
Parfocal Distance (PD)	30.7 mm	30.7 mm	50.5 mm	50.5 mm	80.7 mm	80.7 mm
Mean Spot Size (S) (1/e² Beam Diameter in the Field of Focus)	9 µm	11 µm	17 µm	21 µm	24 µm	29 µm
Curvature (C)	48 µm	48 µm	280 µm	290 µm	220 µm	200 µm
Scan Angle (SA)	7.5°
Max Var. in Optical Path Length (OPL)² from Center to Corner of Image	<1 µm	<1 µm	<3 µm	<5 µm	<10 µm	<5 µm
Transmission Efficiency³ (Beam Energy into Spot Energy)	>93%
Linearity of Conversion from Scan Angle into Spot Position	<2.0%	<2.0%	<1.3%	<1.3%	<2.4%	<2.2%
Lateral Color (Maximum Shift Permitted)	<1.6 µm	<3.2 µm	<9.7 µm	<8.1 µm	<16 µm	<17 µm
Vertical Color, Axial Color, or Chromatic Focal Shift (VC) (Maximum Shift Permitted)¹	<2 µm	<9 µm	<14 µm	<8 µm	<29 µm	<9 µm
Lens Operating Temperature Range (TA)	10 - 50 °C

¹Changes in the EFL due to wavelength are not the same as chromatic focal shift. A change in the EFL indicates a change in the location of the principal plane and hence the magnification of the scan lens. Chromatic focal shift is a wavelength dependent axial deviation in the position of the beam waist.
²The OPL (Optical Path Length) is measured from the center to the the corner of the image and is <50 µm difference from a plane OPL field with 8 mm separation between X and Y scan mirrors.
³ The 1/e² diameter is used for the beam and the spot.

1315 nm OCT Scan Lenses

Item #	LSM02	LSM03	LSM04
Magnification	10X	5X	3X
Center Wavelength	1315 nm
Wavelength Range	±65 nm
Effective Focal Length (EFL)	18.02 mm	35.98 mm	53.99 mm
Lens Working Distance (LWD)	7.5 mm	25.1 mm	42.3 mm
Scanning Distance (SD) (Distance from Pupil Position to Mounting Plane)	15 mm
Pupil Size (1/e²) (EP)	4 mm
Depth of View (DOV)	0.12 mm	0.58 mm	1.15 mm
Variation of Spot Size Over FOV (dS)	11 µm	4 µm	13.5 µm
Distance Between First Lens Surface and Mounting Plate	-4.0 mm	-3.1 mm	-3.0 mm
Field of View (FOV)	4.7 x 4.7 mm	9.4 x 9.4 mm	14.1 x 14.1 mm
Parfocal Distance (PD)	30.7 mm	50.6 mm	80.8 mm
Mean Spot Size (S) (1/e² Beam Diameter in the Field of Focus)	13 µm	25 µm	35 µm
Curvature (C)	48 µm	300 µm	320 µm
Scan Angle (SA)	7.5°
Max Var. in Optical Path Length (OPL)* from Center to Corner of Image	<1 µm
Transmission Efficiency (Beam Energy into Spot Energy) (The 1/e² diameter is used for the beam and the spot)	>93%
Linearity of Conversion from Scan Angle into Spot Position	<2.0%	<1.4%	<2%
Lateral Color (Maximum Shift Permitted)	<6.0 µm	<8.3 µm	<19 µm
Vertical Color, Axial Color, or Chromatic Focal Shift (VC) (Maximum Shift Permitted)	<20 µm	<1 µm	<46 µm
Lens Operating Temperature Range (TA)	10° - 50°C

*) The OPL (Optical Path Length) is measured from the center to the the corner of the image and is <50 µm difference from a plane OPL field with 8 mm separation between X and Y scan mirrors.

Scanning Distance (SD): The SD is the distance between the galvo mirror pivot point and the back mounting plate of the objective. Since the LSM scan lenses are telecentric, the galvo mirror pivot point must be located at the back focal plane of the objective in order to maximize image resolution.
Pupil Size (EP): The size of the EP determines the ideal 1/e² collimated beam diameter that should be used for the beam of light used to image the sample in order to maximize the resolution of the imaging system. All three LSM scan lenses have an EP 4 mm in diameter.
Working Distance (WD or LWD): The distance between the tip of the scan lens housing and the front focal plane of the scan lens is defined as the WD.
Depth of View (DOV): The DOV parameter reported for the LSM scan lenses corresponds to the distance between the front focal plane and a parallel plane where the beam spot size has increased by a factor of the √2.

Field of View (FOV): The FOV is the maximum size of the area on the sample that can be imaged with a resolution equal to or better than the stated resolution of the LSM scan lenses. In order to meet this specification the imaging system must be designed to properly utilize the LSM scan lenses in the system.
Parafocal Distance (PD): The PD is the distance from the scan lens mounting plane to the front focal plane of the LSM scan lenses.
Curvature (C): The curvature is the maximum distance between the front focal surface and an ideal plane.
Scan Angle (SA): The SA is the maximum allowed angle (in the X or Y direction) between the beam and the optical axis of an LSM scan lenses after being reflected off of the galvo mirror.

The plots below show the calculated spot width (in the X and Y directions) as a function of beam position (top axis) or scan angle (bottom axis).

LSM02 Scan Lens

scan lens (lsm02) beam diameter as a function of x-axis scan angle

scan lens (lsm02) beam diameter as a function of y-axis scan angle

LSM03 Scan Lens

scan lens (lsm03) beam diameter as a function of x-axis scan angle

scan lens (lsm03) beam diameter as a function of y-axis scan angle

LSM04 Scan Lens

scan lens (lsm04) beam diameter as a function of x-axis scan angle

scan lens (lsm04) beam diameter as a function of y-axis scan angle

The LSM series of scanning objective lenses was designed to improve the performance of Thorlabs' Swept Source and Spectral Radar OCT imaging systems. This type of objective lens is usually called a scan lens because a laser beam is scanned across the back aperture of the objective lens in order to form the image of the sample. Each position that the laser is scanned over corresponds to one point in the image formed. This approach results in a focal spot on the sample that is not, in general, coincident with the optical axis of the scan lens. In traditional lenses, this would result in the introduction of severe aberrations that would significantly degrade quality of the resulting image. However the LSM series of scan lenses were designed to create a uniform spot size and optical path length for the laser for every scan position, which allows a uniform, high-quality, image of the sample to be formed. Plots that show the spot size as a function of scan position can be seen by looking at the Scan Lens Plots tab.

When designing an imaging system that uses an LSM scan lens it is important to accommodate the Design Wavelength, Parfocal Distance, Scanning Distance, Entrance Pupil, and Scan Angle specifications in order to maximize the image quality (see the Specs tab for LSM scan lens specifications and definitions). For imaging systems with a single galvo mirror the center of the LSM objective's entrance pupil must be coincident with the center of the galvo mirror. If the imaging system uses two galvo mirrors (one to scan in the X direction and one to scan in the Y direction) then the entrance pupil should be located between the two galvo mirrors. It is important to minimize the distance between the two galvo mirrors, because when the entrance pupil and beam steering pivot point are not coincident, the quality of the image is degraded. This is principally due to the variation in the optical path length as the beam is scanned over the sample. Below are schematics for an imaging system containing one and two galvo mirrors.

1d galvo mirror setup

The maximum recommended separation between the two
galvo mirrors is 8 mm.

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