Low-cost Custom Fabrication and Mode-locked Operation of an All-normal-dispersion Femtosecond Fiber Laser for Multiphoton Microscopy

Summary

A method is presented to build a custom low-cost, mode-locked femtosecond fiber laser for potential applications in multiphoton microscopy, endoscopy, and photomedicine. This laser is built using commercially available parts and basic splicing techniques.

Abstract

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped fiber laser is built completely using commercially available parts, including $8,000 in fiber optic and pump laser components, plus $4,800 in standard optical components and extra-cavity accessories. Researchers new to fiber optic device fabrication may also consider investing in basic fiber splicing and laser pulse characterization equipment (~$63,000). Important for optimal laser operation, methods to verify true versus apparent (partial or noise-like) mode-locked performance are presented. This system achieves 70 fs pulse duration with a center wavelength of approximately 1,070 nm and a pulse repetition rate of 31 MHz. This fiber laser exhibits the peak performance that may be obtained for an easily assembled fiber laser system, which makes this design ideal for research laboratories aiming to develop compact and portable fs laser technologies that enable new implementations of clinical multiphoton microscopy and fs surgery.

Introduction

Solid state femtosecond (fs) pulsed lasers are widely used for microscopy and biological research. One typical example is the usage of multiphoton excitation (MPE) fluorescence microscopy, where high peak power and low average power are desired to facilitate the MPE process while minimizing photodamage mechanisms. Many high-performance solid-state lasers are commercially available, and when combined with an optical parametric oscillator (OPO), the laser wavelength can be tuned over a wide range¹. For example, commercial oscillator-OPO systems generate <120 fs pulse durations (typically with an 80 MHz pulse repetition rate) and >1 W aver....

Protocol

1. Splice single mode fibers (SMF)

NOTE: Section 1 consists of general steps to splice SMFs. This is a non-essential, but recommended, step for practicing fiber splices using inexpensive fiber. This step ensures proper performance of the splicing equipment before using more valuable fiber optic materials.

1. Cleave the first fiber.
   1. Strip approximately 30 mm of the fiber with a fiber stripping tool. For fragile fibers (e.g., double clad fibers), a razor blade can be used to carefully peel off the buffer.
   2. Use lint-free tissue with ethanol or isopropanol to clean the stripped fiber. A buzzing sound when wiping t....

Results

It is critical to verify mode-locked operation upon completion of the fiber laser fabrication procedures. Signatures of optimal fs pulse generation and laser stability are as follows: First, the output pulse may be sufficiently characterized by the instrumentation outlined in step 6. The pulse spectrum output from the laser oscillator should be centered near 1,070 nm with the characteristic cat-ear or Batman shape that indicates mode-locking as predicted by numerical simulation of ANDi pu.......

Discussion

The protocols outlined here synthesize know-how and expertise that have been common practice in the laser physics laboratory for decades, but which is frequently unfamiliar to many biomedical researchers. This work attempts to make this ultrafast fiber laser technology more accessible to the broader community. The ANDi fiber laser design is well-established, as first developed in seminal works by Wise and colleagues³. However, implementations of this technology by other groups have sometimes resul.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Adapters, mirrors, posts, mounts, and translational stage (optomechanics)	Thorlabs	TR6-P5 (3x), AD12NT (2x), PFSQ20-03-M01, PFSQ05-03-M01, KMS, KM100C, KM100CL, KM200S, LT1, LT101, UPH2-P5, UPH3-P5 (2x)	Standard optical components
Advanced optical fiber cleaver	AFL	CT-100
Autocorrelator	Femtochrome	FR-103XL/IR/FA/CDA
Beamsplitter mount	Thorlabs	BSH1/M
Factory fusion splicer	AFL	FSM-100P
Fiber collimators	OZ Optics (Canada)	LPC-08-1064-6/125-S-1.6-7.5AS-60-X-1-2-HPC	3x
Fiber-coupled,high-speed photodiode detector	Thorlabs	DET08CFC
Free-space isolator	Thorlabs	IO-5-1050-HP
Free-space isolator	Thorlabs	IO-3D-1050-VLP
Half waveplate	Union Optics (China)	WPZ2312	2x
High power multimode fiber pump module	Gauss Lasers (China)	Pump-MM-976-10
High power pump and signal combiner	ITF Technology (Canada)	MMC02112DF1
Index matching gel	Thorlabs	G608N3
Optical spectrum analyzer	Keysight	Agilent 70951B
Oscilloscope	Keysight	Agilent 54845A
Passive double clad fiber(5/130 μm)	ITF Technology (Canada)	MMC02112DF1	3m, Included with combiner
Polarizing beamsplitter	Thorlabs	PBS253
Quarter waveplates	Union Optics (China)	WPZ4312	2x
Quartz birefringent filter plate	Newlight (Canada)	BIR1060
RF spectrum analyzer	Tektronix	RSA306B
Single mode fiber (6/125 μm)	OZ Optics (Canada)	LPC-08-1064-6/125-S-1.6-7.5AS-60-X-1-2-HPC	1m, Included with collimators
Single mode fiber coupler	AFW (Australia)	FOSC-2-64-30-L-1-H64-2
Transmission diffraction grating 1	LightSmyth	T-1000-1040-3212-94	For compressor
Transmission diffraction grating 2	LightSmyth	T-1000-1040-60x12.3-94	For compressor
Waveplate rotation mount	Thorlabs	RSP1/M	4x
Ytterbium-doped single mode double clad fiber	Thorlabs	YB1200-6/125DC	3m