Semiconductor Guidestar Laser Development

G. J. Fetzer, S. Rako, S. Floyd, L. Hill, N. Woody, T. Baringer,

Areté Associates,

Sandalphon, Cinnabar Optics

and

C. D’Orgeville, The Australian National University

AO4ELT6 - JUNE 9-14, 2019, QUÉBEC CITY, CANADA

Laser Workshop

Outline

• Performance Objectives

• Current Programs

• System Design

• Power 1178 and 589 nm

• Beam Quality

• Laser Mode Selection Principles

• Single Frequency Power

• Locking to Na Resonance

• Summary and Next Steps

2

Areté’s VECSEL GSL System

Characteristic Values and Rationale

Primary l and

Power8-20 W locked to Na(D2a) ~589 nm

Secondary l

and Power

Tunable and lockable at D2b

Dl= 1.7 GHz from D2a

Waveform Continuous Wave

Linewidth 5-50 MHz

Fine Tuning

~1 GHz, continuous

Scan sodium transition to enable line

locking

Gross Tuning~5 GHz, does not need to be continuous

Allow capture of Rayleigh backscatter

Beam Quality M2 < 1.2 Near Diffraction Limited

PolarizationWell defined polarization, contrast ratio

>20 Circular polarization is broadcast

User Interface PC Based GUI

Diagnostics Wavelength and power

Power 110-240 V AC

Water 4-8 slpm flow of cool water

Goal: Design a system that:

• Demonstrates the viability of VECSELs for Guidestar

applications

• Serves as a foundational prototype on which to build

future units

• Lowers acquisition and maintenance costs of GSLs

• Provides utility to astronomy, space situational

awareness, communications, and other applications

VECSEL

Optical

Diagnostics

Pump

Laser

Power

&

Thermal

User

Interface

(PC)

Chiller

Power

D2a/D2b

Power

Water

Optical

Signals

Laser Head

DAC

Guide Star Laser System

ANU Program Snapshot

• Areté is supplying a prototype

VECSEL sodium guidestar laser

to Australian National University

(Celine D’Orgeville).

• ANU is the lead organization of a

consortium consisting of:– Australian Astronomical Observatory

(AAO)

– University of New South Wales

(UNSW),

– The Giant Magellan Telescope

Organization

– EOS Space Systems

– Lockheed Martin Space Systems

• The laser will be installed and

tested on the sky at the Mt

Stromlo Observatory near

Canberra AUS

• Will represent the first on sky

VECSEL GSL demonstration in

the world!

2018 2019

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Tech Demonstrator

Design

Procure and Fab

Integration & Test

Delivery

On-Sky

Areté’s Air Force STTR Program

• Continue to increase 589 nm

VECSEL output powers to

greater than 20W

• Produce a proof-of-concept

system capable of installation at

an observatory for an on-sky test

at 589 nm wavelength

• Deliver a second VECSEL GSL

to an observatory (Starfire

Optical Range)

• Measure on-sky returns from a

VECSEL SGSL at SOR

2019 2020

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Power Scaling

Procure and Fab

Integration & Test

Delivery

On-Sky

Sodium Cell

Excitation

VECSEL System for

On-Sky Demonstration

Laser Head

System

Rack

3 ft

2ft

1.5 ft

1 ft

Weight ~200-250 lbs

Weight ~ 30-40 lbs

Electrical Power ~ 1kW

2ft

1 ft

589 nm

1178 nm

Intracavity

Frequency Doubling /

Output Coupling

High Extraction Efficiency Achieved

with Intracavity Doubling1178 nm 589 nm

• Intracavity doubling is a highly attractive method of converting from the fundamental wavelength to 589 nm

• Low complexity

• Highly efficient

Good Beam Quality at 589 nm

Beam Quality D4σM2x = 1.2M2y = 1.2

M2 Measurement at 589 nm

Tuning and Frequency Selection

Laser Gain Bandwidth

BRF

Etalon

Laser Cavity Modes

Not drawn to scale

Wavelength

Ro

un

dtr

ip G

ain

-to

-Lo

ss R

atio

Gain Bandwidth supports lasing over broad range of frequencies, allowing large number of longitudinal Laser Cavity Modes.BRF provides tunability of center wavelength and coarse frequency selection.Etalon provides fine frequency selection of single cavity mode.Piezo shifts laser modes in wavelength for extra-fine tuning on resonanceNLO not shown but has strong impact on frequency selection

The spectral elements of the cavity are tuned by various methods

including length, angle, and/or temperature

Selected Laser Frequency

Frequency Selection on a Few Scales

Model of Mode Gain/Loss

Plot of individual modes appears “filled

in” because of plot scaleNext Mode Suppression: -40dBLorentzian Linewidth: 2.4 MHz

Etalon FSR

40 nm Scale 5 nm Scale

Etalon FSR

Gain Curve

BRF Curve

Etalon Curve

“Doubled” Sodium Absorption Resonance Plotted Alongside Cavity Modes

Etalon Curve

Laser Modes

Target Waveband

Frequency (GHz)

0

0.2

0.4

0.6

0.8

1

1.2

Arb

Etalon

Sodium

Laser Cavity

-20 -15 -10 -5 0 5 10 15 20

Cavity Modes Up-Close vs Na Spectrum

Cavity longitudinal mode spacing resolved by plot

Sodium D2a/D2b effective spectrum at 1178 nm

0.185 nm Scale

Single Frequency Operation

In the IR, laser “prefers” to run single-frequency.

The lasing mode steals gain from other modes and tends to

be a stable equilibrium.

When intracavity doubling, achieving single-frequency is a

delicate balance.

The lasing mode steals gain from other modes, but also incurs

more intracavity loss due to nonlinear output coupling.

Despite challenges, reasonable output powers at 589 nm single-frequency

on sodium resonance are presently achievable

Performance can an be improved with careful optimization

Work is underway to stabilize frequency while improving extraction efficiency

Wavelength Control Scheme

Optical Electrical

Optical Diagnostics Enclosure

Laser Head Processor Enclosure

Laser Output

100 MHzEO Phase

Modulator

SodiumReference

Cell

HighBandwidthDetector

100 MHzOscillator

Mixer

LowpassFilter

PZT Driver

Microcontroller Servo

CPU

VECSEL with Piezo

Mounted Back Mirror

Amp

Line Locking

Line Locking

• IR wavelength measured while

operating at 589 nm and tuning to

resonance

• Mode-hop-free tuning over sodium

resonance demonstrated

• System can be locked to D2a or

D2b line center or offset from line

centers

Theoretical Error Signal

Measured Error Signal

D2a

Center

Lock

Point

Summary & Next Steps

• SSGSLs are capable of achieving performance characteristics (power, frequency tuning/locking) sufficient for on-sky demonstration

• Presently implementing measures to improve frequency stability and power

• In coming months will begin prototype integration and test– Line locking refinement

– Packaging

– Testing

• Scheduled for Installation 2019 Q3 at Mt. Stromlo, Canberra Australia

Mt. Stromlo Observatory

Laser Head

Electronics &

Control Rack

1.8 m Telescope