CN104716560B - It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light - Google Patents
It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light Download PDFInfo
- Publication number
- CN104716560B CN104716560B CN201310691186.5A CN201310691186A CN104716560B CN 104716560 B CN104716560 B CN 104716560B CN 201310691186 A CN201310691186 A CN 201310691186A CN 104716560 B CN104716560 B CN 104716560B
- Authority
- CN
- China
- Prior art keywords
- raman
- light
- glass tube
- hollow glass
- pond
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Landscapes
- Lasers (AREA)
Abstract
Based on hollow glass tube and the gas excited Raman amplifier of Raman seed light, including a pump laser, a set of beam splitting system, first Raman pond, second Raman pond, a hollow glass tube and one group of Amici prism are carried the invention provides a kind of.Pump light source is divided into two beam pump lights by beam splitting system, wherein the first beam pump light imports first Raman pond by the first two-phase color mirror produces backward Raman seed light；Second beam pump light reaches the second two-phase color mirror by optical path delayed, with backward Raman seed light and carries out conjunction beam simultaneously, is then imported together in a manner of glancing incidence and is placed in progress excited Raman amplification in the hollow glass tube in second Raman pond.Single amplification Raman light is obtained finally by Amici prism component light.Test result indicates that compared with traditional twin-stage connection Raman pond, the present invention can obtain conversion efficiency it is higher be excited amplify Raman light.The present invention can be widely applied to the fields such as military affairs, medical treatment, environmental monitoring.
Description
Technical field
It is particularly a kind of based on hollow glass tube and to carry the gas of Raman seed light the present invention relates to excited Raman amplifier
Body excited Raman amplifier.
Background technology
In recent years, as laser constantly develops in numerous application fields such as traffic, measurement, medical treatment, national defence and industrial or agricultural,
Special optical maser wavelength is developed increasingly to attract much attention.These special optical maser wavelengths can pass through new laser
Operation material produces, and can also pass through gas or optical material（Such as crystal）Nonlinear optical frequency conversion produce.Non-
Linear optics field, stimulated Raman scattering can be used for doing laser emission wavelength specific frequency conversion（Depending on Raman medium
Raman shake/rotate mould frequency）, reach specific optical maser wavelength output.Therefore, stimulated Raman scattering technology is to realize laser wave
The important technical of long conversion.
Physical form according to Raman medium is different, and Raman medium can be divided into solid, liquids and gases.Solid Roman medium
General small volume, Raman concentration of medium is high, therefore its Raman gain and high conversion rate.It is current to have been developed that many kinds of solids Raman is situated between
Matter, application is quite varied, but solid Roman dielectric damages threshold value is low, is not easy to realize that high energy laser exports.Liquid Raman medium
Then due to the volatility of liquid medium, toxicity or unstability the defects of, application is very limited.Comparatively, gas
Body Raman concentration of medium is relatively low, and gain is smaller, but with preferable heat pipe rationality, higher damage threshold（More likely realize big
Energy raman laser exports）, high Raman vibration mould（Big Raman frequency shift）The advantages that with narrow Raman linewidth, therefore also obtained extensively
In-depth study.Conventional gas Raman medium has H2, CH4, O2And N2Deng.
At present, realize that the classical way that laser excited Raman is changed is mainly using gas medium：Pump laser exports
Pump light pass through lens focus, import and stimulated Raman scattering process occur in the one way Raman pond of gassy Raman medium,
Stokes Raman light is produced, Raman amplifiction light is then obtained by collimation lens, prismatic decomposition again.In the process, only
Laser power density can reach stimulated Raman scattering threshold value in a bit of region near the focal position of condenser lens；
That is frequency conversion of the stimulated Raman scattering realization to pump light can just occur only in this region.Therefore pump light
With the effective interaction region very little of Raman medium, it is difficult to obtain higher Raman transformation efficiency.
In addition, the excited Raman threshold value generally said refers to realize from spontaneous by stimulated Raman scattering in Raman vibration pond
Noise caused by Raman scattering is amplified to minimum pumping light intensity required during Observable degree.Classical one way Raman pond swashs
Light wavelength conversion, it is that spontaneous Raman scattering provides seed light for follow-up stimulated Raman scattering.Further, since gas medium
Particle concentration is small, and this means that traditional gas gain coefficients comparison is small, and stimulated Raman scattering threshold value can be higher.Therefore,
There has been proposed a kind of raman amplifier for carrying Raman seed light, then the effect of spontaneous Raman scattering can be ignored, its
Stimulated Raman scattering process actually avoids the need for threshold value.So, it is higher to solve gas stimulated Raman scattering threshold value
The also all increases of the effective interaction region of technical barrier, pump light and Raman seed light, but the pump light away from focal point
Power density declines quickly, it is difficult to continue excited Raman amplification process, i.e. effective interaction region has still limited,
Therefore still it is difficult to obtain higher Raman transformation efficiency.
The content of the invention
The present invention is solves technical problem present in above-mentioned background technology, there is provided one kind based on hollow glass tube and from
Gas excited Raman amplifier with Raman seed light, it can substantially increase the effective interaction of pump light and Raman seed light
Length, and the power density of pump light is improved, significantly improve Raman phototranstormation efficiency.
The technical solution of the present invention is as follows：
It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light, including a pumping laser
Device, a set of beam splitting system, first Raman pond, second Raman pond, a hollow glass tube and one group of Amici prism, its feature
It is：The pump light of pump laser output is divided into two beam pump lights by beam splitting system；First beam pump light passes through first
Raman pond produces backward stokes light as Raman seed light, and backward stokes light is as Raman seed light and the second beam pump
Pu light is imported progress Raman light in the hollow glass tube being placed in second Raman pond in a manner of glancing incidence and is excited to amplify simultaneously.
Wherein, the first beam pump light is focused on by the reflection of the first two-phase color mirror and the first condenser lens and imports first Raman
Pond, backward Stokes Raman seed light is produced therewith, the Raman seed light returns by original optical path, it is saturating to sequentially pass through the first focusing
Mirror, the first two-phase color mirror, the second condenser lens and the second two-phase color mirror enter in the hollow glass tube built in second Raman pond,
As Raman seed light；Second beam pump light successively carries out light by the first high reflective mirror, tertiary focusing lens and the second high reflective mirror
Road postpones, and amplifies pump light as excited Raman, reaches the second two-phase color mirror simultaneously with backward Stokes Raman seed light, so
Spatially reach with backward Stokes Raman seed light and overlap by the reflection of the second two-phase color mirror, after together with glancing incidence side
Formula imports the hollow glass tube being placed in second Raman pond and carries out excited Raman amplification；Finally, hollow glass tube output port
Output light successively by collimation lens and Amici prism group, remaining pump light P and amplification Raman light S1 are separated, so as to
To single amplification Raman light.
Wherein, described Raman seed light and the second beam pump light pass through the second condenser lens and tertiary focusing lens respectively
Spatially focus on the input port for the hollow glass tube being placed in second Raman pond.
Wherein, described Raman seed light and the second beam pump light reach the simultaneously in time by respective light path respectively
In two two-phase color mirrors and carry out conjunction beam.
Wherein, described Raman seed light be the first beam pump light caused by first Raman pond after to Stokes
Excited Raman light, its beam quality are better than pump light.
Wherein, it is incidence window on the left of described first Raman pond, right side is exit window, and two windows are set to the
One quartz window piece and the second quartz window piece, the first beam pump light after the first condenser lens first pass through first Raman pond
Incidence window is transmitted away by exit window again, and caused backward Stokes Raman seed light is by original optical path through incidence end the
One diaphragm returns.
Wherein, it is incidence window on the right side of described second Raman pond, left side is exit window, and two windows are set to the
After three quartz window pieces and the 4th quartz window piece, Raman seed light and the second beam pump light close beam in the second two-phase color mirror
Mixed light first passes through second Raman pond incidence window, imported together in a manner of glancing incidence be placed in it is hollow in second Raman pond
Excited Raman amplification process occurs for glass tube, and then remaining pump light P and amplification Raman light S1 are transmitted by exit window
Go out.
Wherein, described beam splitting system is made up of 3 high reflective mirrors, 2 1/2 wave plates and 2 polarization spectro pieces, and pump light is first
By the one 1/2 wave plate, S and P polarization light light intensity are continuously changed by the angle for adjusting 1/2 wave plate optical axis, then it is inclined by first
The light splitting piece that shakes is split, and the light beam of lower section transmission is the first beam pump light, and the light beam of right reflection is the second beam pump light.
Wherein, output end of the focused spot of described collimation lens in the hollow glass tube being placed in second Raman pond
Mouthful.
Wherein, the first described two-phase color mirror is high to pump light anti-to be high thoroughly to Raman light；Second two-phase color mirror is to drawing
Graceful light is high thoroughly, high to pump light anti-.
For stable state stimulated Raman scattering process, under the conditions of small-signal gain（The strong loss of i.e. negligible pump light）,
Amplify Raman spectra length to meet：
Is(z)=Is(0)exp(gIpz)
Wherein：Is(z) it is the amplification Raman light intensity increased, Is(0) it is initial Raman seed light intensity, g is steady-state gain system
Number, IpFor pumping light intensity, z is Raman light and pump light effective interaction length.During above formula shows stimulated Raman scattering,
It is directly proportional to Raman seed light intensity to amplify Raman light intensity, with interaction length z exponentially relation with increase.
With it is traditional carry the stimulated Raman scattering wavelength conversion of seed light compared with, the excited Raman amplification according to the present invention
There are following two obvious advantages：
1）, because the second beam pump light is imported and is bound in hollow glass tube in a manner of glancing incidence, then pump light exists
Power density in whole hollow glass tube is larger, is very beneficial for carrying out stimulated Raman scattering amplification process.
2）, be bound in hollow glass tube due to Raman seed light and the second beam pump light, effective phase of two-beam
Interaction length（Theoretically whole hollow glass length of tube）Greatly increase so that pump light is easily largely consumed, and is turned
Raman light is turned to, i.e. quantum conversion will greatly improve.
Brief description of the drawings
Fig. 1 be the present invention based on hollow glass tube and carry Raman seed light gas excited Raman amplifier architecture signal
Figure.
Wherein 1- pump lasers, 2- beam splitting systems, the two-phase color mirrors of 3- first, the condenser lenses of 4- first, the quartz of 5- first
Diaphragm, first Raman pond of 6-, 7- the second quartz window pieces, the condenser lenses of 8- second, the high reflective mirrors of 9- first, 10- the 3rd 1/2
Wave plate, 11- tertiary focusing lens, the high reflective mirrors of 12- second, the two-phase color mirrors of 13- second, the quartz window pieces of 14- the 3rd, 15,24- fills
Air valve, second Raman pond of 16-, 17,23- air gauges, the quartz window pieces of 18- the 4th, 19- hollow glass tubes, 20,21- it is empty
Heart glass pipe holder, 22- collimation lenses, 25- Amici prism groups.
Fig. 2 is to be excited to amplify the curve map that the quantum conversion of Raman light changes with the second beam pumping light intensity（This hair
Bright raman amplifier and traditional twin-stage connection Raman pond excited Raman）
Embodiment
In detail as shown in Figure 1.As seen from the figure, the present invention based on hollow glass tube and carries the gas of Raman seed light and is excited to draw
Graceful amplifier includes：One pump laser 1, a set of beam splitting system 2 that pump light is divided into two beams, the high anti-and Raman of pump light
High the first saturating two-phase color mirror 3 of light, the first condenser lens 4 of the first beam pump light, first Raman pond 6 and its milli of both ends of the surface 10
The first quartz window piece 5, the second quartz window piece 7 of 30 millimeters of diameter are respectively set on the thick sealing flange of rice, installed in first drawing
Air gauge 23 and charging valve 24 on graceful pond 6, the second condenser lens 8 of backward Raman seed light, for the second beam pump light
Optical path delayed the first high reflective mirror 9, the wave plate 10 of the second high reflective mirror the 12, the 3rd 1/2, the tertiary focusing lens of the second beam pump light
11, by Raman seed light and the second two-phase color mirror 13 of the second beam pumping combiner, second Raman pond 16 and its both ends of the surface 10
The 3rd quartz window piece 14, the 4th quartz window piece 18 of 30 millimeters of diameter are respectively set on millimeters thick sealing flange, is built in second
Hollow glass tube 19 and its both ends support 20 and 21 in individual Raman pond, air gauge 15 on second Raman pond 16 and
Charging valve 17, the collimation lens 22 of output amplification Raman light, one group of Amici prism 25.Wherein, first Raman pond 6 and second
The main body of individual Raman pond 16 is 26 millimeters of internal diameter, the stainless steel tube of 3 millimeters of wall thickness, and length is respectively 300 millimeters and 1200 millis
Rice；Hollow glass tube 19 is quartz material, and its internal diameter is 1.5 millimeters, and length is 580 millimeters.
Specifically, the pump light that pump laser 1 exports is divided into two beam pump lights by beam splitting system 2.Wherein first
Beam pump light passes through the first two-phase color mirror 3（It is high to pump light anti-, it is high to Raman light saturating）Reflection and the first condenser lens 4（Focal length
For 250 millimeters）Left side incidence window through first Raman pond 6 after focusing is imported in the Raman pond 6, produces beam quality therewith
Preferably backward Stokes Raman seed light, the Raman seed light by original optical path return, successively by the first condenser lens 4,
First two-phase color mirror 3, the second condenser lens 8（Focal length is 1000 millimeters, and focus is in the hollow glass built in second Raman pond 16
The input port of pipe 19）Enter the input port of hollow glass tube 19 in a manner of glancing incidence with the second two-phase color mirror 13, as drawing
Graceful seed light, right side exit window of the remaining first beam pump light through first Raman pond 6 transmit away；Second beam pump light
Tertiary focusing lens 11 that are optical path delayed and passing through centre are successively carried out by the first high reflective mirror 9, the second high reflective mirror 12（Focal length is
1000 millimeters, input port of the focus in hollow glass tube 19）Focus on, amplify pump light as excited Raman, with backward Raman
Seed light reaches the second two-phase color mirror 13 simultaneously, and hollow glass is imported in a manner of glancing incidence in the lump after then spatially carrying out conjunction beam
Glass pipe 19 carries out excited Raman amplification.Finally by collimation lens 22（Focal length is 300 millimeters, and focus is in hollow glass tube 19
Output port）Remaining pump light P and amplification Raman light S1 are separated with Amici prism group 25, drawn so as to obtain single amplification
Graceful light.
The embodiment of the present invention, using U.S. Continuum Nd:YAG laser fundamental frequency light 1064nm as pump light, with
H2For stimulated Raman scattering medium, the generation and its excited Raman for realizing its single order Stokes 1900nm Raman light seed light are put
Greatly.First Raman pond 6 and second Raman pond 16 inject 3.5MPa hydrogen.Carried out according to above-mentioned experimental procedure based on sky
Heart glass tube and the H for carrying Raman seed light2Excited Raman amplification test.Carrying out traditional twin-stage connection Raman amplifiction experiment
When, the hollow glass tube built in second Raman pond in above-mentioned experimentation need to only be taken out, fill hydrogen medium again, other
Light path is constant.Finally above-mentioned two excited Raman light light extraction efficiency is compared and analyzed.
Fig. 2, which is depicted, to be excited to amplify two songs that the quantum conversion of Raman light changes with the second beam pump light light intensity
Line chart, traditional twin-stage connection raman amplifier and the raman amplifier of the present invention are corresponded to respectively.As can be seen from Figure 2：When second
When beam pump light is 63.4mJ, the former Raman quantum conversion is 22%, and the latter's quantum conversion is 32%, i.e. quantum turns
Change efficiency and improve 45%.Generally speaking, the former maximum Raman quantum conversion is 23%, and the latter's maximum quantum transformation efficiency is
35%, i.e. maximum quantum transformation efficiency improves 52%.Test result indicates that：Join Raman pond excited Raman phase with traditional twin-stage
Than the excited Raman amplification efficiency according to the present invention is significantly improved.
Claims (6)
- Based on hollow glass tube and the gas excited Raman amplifier of Raman seed light, including a pumping laser are carried 1. a kind of Device (1), a set of beam splitting system (2), first Raman pond (6), second Raman pond (16), a hollow glass tube (19) and one Group Amici prism (25).The pump light of pump laser (1) output is divided into two beam pump lights, its feature by beam splitting system (2) It is：First beam pump light passes through backward Stokes Raman seed light and the second beam pump light caused by first Raman pond (6) Progress Raman light in the hollow glass tube (19) being placed in second Raman pond (16) is imported in a manner of glancing incidence simultaneously to be excited to put Greatly.
- 2. gas excited Raman amplifier according to claim 1, it is characterised in that：First beam pump light is focused on by the first two-phase color mirror (3) reflection and the first condenser lens (4) and imports first Raman pond (6) backward Stokes Raman seed light, is produced therewith, and the Raman seed light is returned by original optical path, sequentially passes through the first focusing Lens (4), the first two-phase color mirror (3), the second condenser lens (8) and the second two-phase color mirror (13) enter hollow glass tube (19) It is interior, as Raman seed light；Second beam pump light successively carries out light path by the first high reflective mirror (9), tertiary focusing lens (11), the second high reflective mirror (12) Delay, amplifies pump light as excited Raman, and the second two-phase color mirror (13) is reached simultaneously with backward Stokes Raman seed light, Then through the second two-phase color mirror (13) reflect with backward Stokes Raman seed light spatially reach coincidence after together with enter Excited Raman amplification is carried out in hollow glass tube (19)；The output light of hollow glass tube (19) output port successively by the 4th quartz window piece (18), collimation lens (22) and Amici prism group (25) separates remaining pump light P and amplification Raman light S1, so as to obtain single amplification Raman light.
- 3. gas excited Raman amplifier according to claim 1, it is characterised in that：Described Raman seed light and the second beam pump light is existed by the second condenser lens (8) and tertiary focusing lens (11) respectively Spatially focus on the input port of hollow glass tube (19).
- 4. gas excited Raman amplifier according to claim 1, it is characterised in that：It is incidence window on the right side of second described Raman pond (16), left side is exit window, and two windows are set to the 3rd stone English diaphragm (14) and the 4th quartz window piece (18), Raman seed light and the second beam pump light are in the second two-phase color mirror (13) Close the mixed light after beam and first pass through second Raman pond (16) incidence window, imported in a manner of glancing incidence be placed in second drawing together Excited Raman amplification process occurs for the hollow glass tube (19) in Man Chi (16), then remaining pump light P and amplification Raman light S1 is transmitted away by exit window.
- 5. gas excited Raman amplifier according to claim 1, it is characterised in that：Described beam splitting system (2) is made up of 3 high reflective mirrors, 2 1/2 wave plates and 2 polarization spectro pieces, and pump light first passes through One 1/2 wave plates, S and P polarization light light intensity are continuously changed by the angle for adjusting 1/2 wave plate optical axis, then by the first polarization spectro Piece is split, and the light beam of lower section transmission is the first beam pump light, and the light beam of right reflection is the second beam pump light.
- 6. gas excited Raman amplifier according to claim 2, it is characterised in that：The focused spot of described collimation lens (22) is defeated the hollow glass tube (19) being placed in second Raman pond (16) Exit port.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN201310691186.5A CN104716560B (en) | 2013-12-13 | 2013-12-13 | It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN201310691186.5A CN104716560B (en) | 2013-12-13 | 2013-12-13 | It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN104716560A CN104716560A (en) | 2015-06-17 |
| CN104716560B true CN104716560B (en) | 2018-02-23 |
Family
ID=53415639
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN201310691186.5A Expired - Fee Related CN104716560B (en) | 2013-12-13 | 2013-12-13 | It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN104716560B (en) |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4239995A (en) * | 1979-02-27 | 1980-12-16 | Westinghouse Electric Corp. | Metal vapor Raman frequency shifter |
| US8073017B1 (en) * | 2009-02-23 | 2011-12-06 | The United States Of America As Represented By The Secretary Of The Navy | System and method for generating eye-safe high intensity optical pulses with two backward-shifting raman cells |
-
2013
- 2013-12-13 CN CN201310691186.5A patent/CN104716560B/en not_active Expired - Fee Related
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4239995A (en) * | 1979-02-27 | 1980-12-16 | Westinghouse Electric Corp. | Metal vapor Raman frequency shifter |
| US8073017B1 (en) * | 2009-02-23 | 2011-12-06 | The United States Of America As Represented By The Secretary Of The Navy | System and method for generating eye-safe high intensity optical pulses with two backward-shifting raman cells |
Also Published As
| Publication number | Publication date |
|---|---|
| CN104716560A (en) | 2015-06-17 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN103855602A (en) | Gas stimulated Raman amplifier with Raman seed light | |
| CN101211088B (en) | Single crystal tunable broad band noncolinear femtosecond light parameter magnification method and apparatus | |
| JP6050684B2 (en) | Widely tunable optical parametric oscillator | |
| CN103022888B (en) | Alkali metal steam laser of polarized optical pumping | |
| CN106911060B (en) | The high-efficiency high power mid-infrared laser device of Wavelength tunable | |
| US8284478B2 (en) | Device for increasing the spectral bandwidth of optical pulses as well as an arrangement and a method for reducing the duration of optical pulses with the use of such a device | |
| CN103036140B (en) | A kind of blue-violet laser based on frequency multiplication vapour of an alkali metal laser | |
| US10483709B2 (en) | Sub-nanosecond broad spectrum generating laser system | |
| Kraemer et al. | High-power femtosecond infrared laser source based on noncollinear optical parametric chirped pulse amplification | |
| JP2010026027A (en) | Ultraviolet laser device | |
| CN110021873B (en) | Laser based on cascade stimulated scattering | |
| Lago et al. | Single-mode narrow-bandwidth temporally shaped nanosecond-pulse ytterbium-doped fiber MOPA for a large-scale laser facility front-end | |
| CN104821482A (en) | Quasi-continuous 589nm laser based on crystal Raman technology and application thereof | |
| CN110535017A (en) | All-solid-state yellow laser based on codope crystal | |
| JP2008299155A (en) | Contrast enhancing method of high intensity laser | |
| Dawson et al. | Multi-watt 589nm fiber laser source | |
| CN104716560B (en) | It is a kind of based on hollow glass tube and to carry the gas excited Raman amplifier of seed light | |
| CN108711732B (en) | All-fiber low repetition frequency parametric oscillator for stimulated Raman scattering | |
| CN111725695A (en) | All-solid-state ultraviolet laser capable of simultaneously generating 289nm laser and 299nm laser | |
| Wang et al. | Shock compression-induced enhancement of stimulated Raman scattering in heavy water | |
| CN106558829B (en) | 589nm laser based on the double Raman media of total chamber and laser and frequency | |
| Tarasov et al. | Subnanosecond Nd: YAG laser with multipass cell for SBS pulse compression | |
| Losev et al. | THL-100 multi-terawatt laser system of a visible range | |
| CN107293935A (en) | A kind of device for producing short wavelength deep laser | |
| Long et al. | 1 kHz, 96 W pulse-burst picosecond laser system with six equal spacing and amplitude pulses |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| C06 | Publication | ||
| PB01 | Publication | ||
| C10 | Entry into substantive examination | ||
| SE01 | Entry into force of request for substantive examination | ||
| GR01 | Patent grant | ||
| GR01 | Patent grant | ||
| CF01 | Termination of patent right due to non-payment of annual fee |
Granted publication date: 20180223 Termination date: 20211213 |
|
| CF01 | Termination of patent right due to non-payment of annual fee |