ITU Workshop on Quantum Information Technology for Networks

Fiber-based Optical frequency comb

National Time Service Center（NTSC）

Haifeng JIANG

7th June 2019 · Shanghai

Outline

Background Introduction

Er:fiber based mode-locked laser

Frequency control techniques

Application of the Er:fiber based comb @ NTSC

2

2005 Nobel Prize

Cs Atomic clock

John L Hall JILA—NIST&CU

Theodor W. HänschMPQ

Ca frequency standard

Invention of the optical frequency comb paves the way of precise measurement (phase and frequency) in optical domain.

3

4

In 2010, Hansch pointed out that optical frequency combs had been used in more than 100 applications

Optical frequency standards

Ultraviolet combFrequency comparison Fundamental constant measurement

Optical frequency links ranging Primary frequency standard

High precision radar

1. Femtosecond mode-locked laser

Ti: sapphire (lowest noise)

Er:fiber (best robustness)

Yb:fiber (good power conversion)

2. Control of comb`s frequencies:

repetition rate (fr)

Carrier envelope offset frequency

(fceo)

Optical frequency comb is a frequency controlled femtosecond mode-locked laser.

Two steps forward a comb.

5

What is the optical frequency comb?

NPR ~ nonlinear polarization rotation, a passive mode-locking technique

6

Short time (fs)Inte

nsity

k = 1

k = 7

Notice that the weak pulses are suppressed, and the strong pulse shortens and is amplified.

k = 2

Input pulse PC PCFiber Output

pulse

Intensity-dependent rotation of light

Polarization Controller Polarizer

Saturable Absorber

Mode-locking mechanism

NPR Er:fiber optical frequency combNPR ~ nonlinear polarization rotation, a passive mode-locking technique

7

XX

WDM+ISO

Er:fiber

CO

CO

SMF-28

λ/4

λ/2

PBS

EOM

λ/2

λ/4

V

Pump Laser

Output

PZT

V

Kerr effect -> NPR

NPR + Polarization selection= SAPump Laser： 976nm，Power~1W；

WDM： Wavelength Division Multiplexer；

Er:fiber：Liekki ER110, ~44cm；

SMF-28：34cm,；

EOM： LiNbO3, 8mm；

PZT： Piezotransducer，20mm；

Net dispersion ~ -2000 fs2；

NPR Er:fiber optical frequency combNPR ~ nonlinear polarization rotation, a passive mode-locking technique

8

1500 1550 1600 16500.0

0.1

0.2

0.3

0.4

0.5

0.6

Inte

nsity

(mW

/nm

)

wavelength(nm)

-450 -300 -150 0 150 300 4500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

scale time (fs)

Output power：~180 mW (ƞ ~ 18%);

10 dB bandwidth：~ 130 nm;

Repetition rate：~ 232 MHz;

Pulse duration：~ 60 fs；

Autocorrelation

Optical spectrum

Peak power is more than 10 kW

Hybride Er:fiber optical frequency comb

9

Reliability of optical frequency combs is also importance for applications.

NPR (left, low noise) +

NALM (right, robust)

= Hybride fs laser (Self-start)

Self-start mode-locking;

Large polarization tuning

dynamic range;

Large power tuning dynamic

range.

Broadband frequency control.

Y. Zhang, et al. (2017). "Robust optical-frequency-comb based on the hybrid mode-locked Er:fiber femtosecond laser." Optics Express 25(18): 21719-21725.

Hybride Er:fiber optical frequency comb

10

Laser Journal of China 41(8)，0802004, 2014; Opt. Exp. 25, 21719, 2017

100 101 102 103 104 105

-150

-140

-130

-120

-110

-100

-90

RIN

(dBc

/Hz)

Frequency (Hz)

NPR+MALM NPR Menlo FC1500

Hybride Er:fiber optical frequency comb

11

Optica 3 (12), 1381, 2016

Full PM fiber mode-locked laser (Figure-9)

Figure-9 mode-locked laser

Phase shifter

Phase shifter

Suitable for space application!

Nonlinear amplification

loop mirror (NALM)

mode-locking mechanism

Sagnac Interferometer + kerr effect

Menlo system Inc. NTSC

Turn-key system

13

PZTLaser Cavity

Environmental changes/pump noise /amplified spontaneous emission

cavity length

EOM

ƒrep

Frequency control broad bandwidth tight phase lock with less residual noise large range long term phase lock

Pump laser

dispersion

ƒceo

Environmental changes/pump noise /amplified spontaneous emission

Laser Cavity

repetition rate determined by laser cavity lengthoffset frequency dependent on dispersion in the laser cavity

Frequency control of OFC

fceo detection and control

14

Signal to noise ratio of fceo is a limitation of Robustness of optical combs

νN=fceo+N fr

fs laser

2n fr+fceo

n fr+fceo2n fr+2fceo

15

～30mW

Reference narrow line-width laser @ 1550 nm

λ/2

λ/2 Polarizer

IF LO

Frequency counter

PLL

PLL

÷40

Ref. frequency

EOM

PZT

200 300 400 500 600 700-90

-80

-70

-60

-50

-40

-30

-20

-10

Inte

nsity

(dBm

)

Frequency (MHz)

RBW=300 KHz Span=600 MHz

fast

slow

Repetition rate control

Results of frequency controlEvaluation of frequency control ability

16

-100 -50 0 50 100-80

-60

-40

-20

0

Inte

nsity

(dBm

)

Frequency (kHz)

RBW=1 kHz Span=200 kHz

-100 -50 0 50 100

-50

-40

-30

-20

-10

0

10

Inte

nsity

(dBm

)

Frequency (kHz)

RBW=1 KHz Span=200 KHz

A comb teeth and fceo are frequency stabilized simultaneously

Gain bumpsWith cross-talk

Comb teeth fceo

17

Results of frequency control

fceo

fr

Applications of our comb systems

18

696 697 698 699 700 70102468

101214

Inte

nsity

(mW

/nm

)

wavelength (nm) 210 240 270-100

-80

-60

-40

-20

Inte

nsity

(dBm

)

Frequency (MHz)

RBW=300 kHz Sr Optical clock @ NTSC

~ 1 μW/mode @ 698 nm

Y. Zhang，L. Yan，H. Jiang，S. Zhang, Journal of time and frequency， 40(3), 2017

Applications of our comb systems

19

Ultra stable photonic microwave

generator

3X

Thank you for your attention!

The end!

20

News for metrologyThe International System of Units (SI)

cd

s

m

mol

K

kg

A

Up to last month

Definition of the meter, the ampere and the candela depend on the definition of the second

cd

s

m

mol

K

kg

A

From 20th May 2019

All definitions except that of the mole depend on the definition of the second

“Never measure anything but frequency”

22

Arthur L. Schawlow winner of the 1981

Nobel Prize for Physics

Quoted by Theodor W. Hansch in his

2005 Nobel Prize Lecture