Radio-frequency Transfer on the 112Km-long Urban Optical Fiber Network

Myoung-Sun Heo

Korea Research Institute of Standards and Science

Oct. 31 2015

with support from

Outline

• Motivations of RF transfer over fiber networks

• Characteristics of the fiber network in use

• Schematics of RF transfer

• Results and discussions

• Future works

Motivations of “RF” transfer over the fibers

• Most accurate frequency disseminations are done in the optical frequency regime, but…..

• RF transfer has advantages against optical frequency transfer

✓ It does not require frequency combs to obtain of RF signals.

➡ directly applicable to instruments in other institutes

✓ It assures a long-term operation which is essential for the clock comparison

✓ It can reach instability ~10-17 in a day.

✓ In Korea, we need to use wireless method to compare our clocks with those in other countries. Fortunately VLBI (for space geodesy) and SLR are located in the same place which is 30 km away from our institute. RF transfer will provide them with way to evaluate and backup their commercial clocks and on our side they can be used for ATFT.

KREONETKorea Research Environment Open Network

VLBI & SLR

30 km

56 km

KREONET

VLBI & SLR

(Daejeon) Local Remote

(Cheongju)

56 km

18.1

dB

17.7

dB

30 km

56 km

Korea Research Environment Open Network

reflections from PC-type connectors

Schematics

Challenges

1. Rather high attenuation : use EDFAs

2. Unwanted large reflections : use lasers with different λ3. EDFA’s had to be installed in the detection path : FBA

unwanted reflection (~5dB below)

Laser1555nm

1GHzVCXO

1GHzreference

10MHz

phase detector

Laser1551nm

1GHztransferred

OBFEDFA

1GHzVCXO

100MHz

1GHz5MHz

servo

EAM:LD

PD

Phase detector (compensation scheme)

1GHzVCXO

100MHz

1GHz5MHz

servo

EAM:LD

PD

/10 /10 /2

EAM:LD

PD5MHz out

M.Kumagai et al

Alternatives :

First, stabilise the optical path length of the finer using the optical frequency transfer technique with optical compensator which has advantages of compact electronics using digital electronics, simpler scheme, efficient elimination of unwanted reflections etc. in Then we may able to disseminate accurate RF signals on this stabilised path.

Results

Without OBF

With OBF

1.0E-17

1.0E-16

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1 10 100 1000 10000 100000

Results

-130

-125

-120

-115

-110

-105

-100

-95

-90

-85

-80

1 10 100 1000 10000 100000 1000000

• 0.5 Hz NEBW• Short-term stability is improving

with better amplifiers • Long-term stability is the same

Allan deviation

Phase noise

• Loop bandwidth ~ 250 Hz• Need to suppress servo bump by

modifying PID servo such as including D component

1.2E-16

Future works

• Better short-term stability : Use FBA’s to get more gain and SNR

• Better long-term stability

• use the optical compensator to handle with large fluctuations

• OR, stabilise the optical path length of the finer using the optical

frequency transfer technique with optical compensator which has

advantages of compact electronics using digital electronics, simpler

scheme, efficient elimination of unwanted reflections etc. in Then we

may able to disseminate accurate RF signals on this stabilised path

• Try to establish the finer link between KRISS and VLBI